View source for Aggression

{{Short description|Social interaction aiming at inflicting harm or unpleasantness}}
{{Redirect2|Aggressive|Aggressive behavior|other uses|Aggressive (disambiguation)|and|Aggression (disambiguation)|and|Aggressive Behavior (journal)}}
{{Use dmy dates|date=December 2019}}
[[File:Agression contre Fallières - détail (Petit Parisien illustré, 1909-01-10).jpeg|upright|thumb|Depiction of French President [[Armand Fallieres]]'s assault by a waiter named Jean Mattis]]
[[File:Panthera tigris altaica 28 - Buffalo Zoo (1).jpg|thumb|upright|Many mammals, such as the [[tiger]], bare their teeth as a sign of aggression—a form of [[aposematism]]. ]]
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'''Aggression''' is behavior aimed at opposing or attacking something or someone. Though often done with the intent to cause harm, some might channel it into creative and practical outlets.<ref>{{cite journal |last1=Maremmani |first1=Icro |last2=Avella |first2=Maria T. |last3=Novi |first3=Martina |last4=Bacciardi |first4=Silvia |last5=Maremmani |first5=Angelo G.I. |title=Aggressive Behavior and Substance Use Disorder: The Heroin Use Disorder as a Case Study |journal=Addictive Disorders & Their Treatment |date=September 2020 |volume=19 |issue=3 |pages=161–173 |doi=10.1097/ADT.0000000000000199 }}</ref> It may occur either reactively or without provocation. In humans, aggression can be caused by various triggers. For example, built-up [[frustration]] due to blocked goals or perceived disrespect.<ref>{{cite journal |last1=DeBono |first1=Amber |last2=Muraven |first2=Mark |title=Rejection perceptions: feeling disrespected leads to greater aggression than feeling disliked |journal=Journal of Experimental Social Psychology |date=November 2014 |volume=55 |pages=43–52 |doi=10.1016/j.jesp.2014.05.014 }}</ref> Human aggression can be classified into direct and indirect aggression; while the former is characterized by physical or verbal behavior intended to cause harm to someone, the latter is characterized by behavior intended to harm the social relations of an individual or group.<ref name=":3">{{cite journal |doi=10.1016/j.physbeh.2015.02.053 |pmid=25749197 |title=Behavioural, hormonal and neurobiological mechanisms of aggressive behaviour in human and nonhuman primates |journal=Physiology & Behavior |volume=143 |pages=121–35 |year=2015 |last1=De Almeida |first1=Rosa Maria Martins |last2=Cabral |first2=João Carlos Centurion |last3=Narvaes |first3=Rodrigo }}</ref><ref name=":4">{{cite journal |last1=Miczek |first1=Klaus A. |last2=de Almeida |first2=Rosa M. M. |last3=Kravitz |first3=Edward A. |last4=Rissman |first4=Emilie F. |last5=de Boer |first5=Sietse F. |last6=Raine |first6=Adrian |title=Neurobiology of Escalated Aggression and Violence |journal=The Journal of Neuroscience |date=31 October 2007 |volume=27 |issue=44 |pages=11803–11806 |doi=10.1523/JNEUROSCI.3500-07.2007 |pmid=17978016 |pmc=2667097 }}</ref><ref>{{cite web|url=https://www.rcn.org.uk/magazines/Advice/2022/October/Microaggressions-calling-out-racism-in-the-workplace|title=Micro refers to its subtle delivery not its impact|quote=Microaggressions are something that somebody says to you; they can be a way that you've been made to feel. They are often subtle behaviours, but their effects are far from subtle |first=Lorna |last=Hollowood|year=2022|publisher=[[Royal College of Nursing]]|website=rcn.org.uk}}</ref>

In definitions commonly used in the [[social sciences]] and [[behavioral science]]s, aggression is an action or response by an individual that delivers something unpleasant to another person.<ref>Buss, A. H. (1961). The psychology of aggression. Hoboken, NJ: John Wiley.{{page needed|date=February 2017}}</ref> Some definitions include that the individual must intend to harm another person.<ref name="auto">{{cite journal |doi=10.1146/annurev.psych.53.100901.135231 |pmid=11752478 |title=Human Aggression |journal=Annual Review of Psychology |volume=53 |pages=27–51 |year=2002 |last1=Anderson |first1=Craig A. |last2=Bushman |first2=Brad J. }}</ref>

In an interdisciplinary perspective, aggression is regarded as "an ensemble of mechanism formed during the course of evolution in order to assert oneself, relatives, or friends against others, to gain or to defend resources (ultimate causes) by harmful damaging means. These mechanisms are often motivated by emotions like fear, frustration, anger, feelings of stress, dominance or pleasure (proximate causes). Sometimes aggressive behavior serves as a stress relief or a subjective feeling of power."<ref>{{Cite book|last=Wahl|first=Klaus|title=The Radical Right. Biopsychosocial Roots and International Variations|publisher=Palgrave Macmillan|year=2020|isbn=978-3-030-25130-7|location=London|pages=47|oclc=1126278982}}</ref><ref>{{Cite book|last=Wahl|first=Klaus|title=Aggression und Gewalt. Ein biologischer, psychologischer und sozialwissenschaftlicher Überblick|publisher=Spektrum Akademischer Verlag|year=2013|isbn=978-3-8274-3120-2|location=Heidelberg|pages=2|oclc=471933605}}</ref> [[predation|Predatory]] or defensive behavior between members of different species may not be considered aggression in the same sense.

Aggression can take a variety of forms, which may be expressed physically, or communicated [[Verbal aggressiveness|verbally]] or non-verbally, including: anti-predator aggression, defensive aggression (fear-induced), predatory aggression, dominance aggression, inter-male aggression, resident-intruder aggression, maternal aggression, species-specific aggression, sex-related aggression, territorial aggression, isolation-induced aggression, irritable aggression, and brain-stimulation-induced aggression (hypothalamus). There are two subtypes of human aggression: (1) controlled-instrumental subtype (purposeful or goal-oriented); and (2) reactive-impulsive subtype (often elicits uncontrollable actions that are inappropriate or undesirable). Aggression differs from what is commonly called [[assertiveness]], although the terms are often used interchangeably among laypeople (as in phrases such as "an aggressive salesperson").<ref>Akert, R.M., Aronson, E., & Wilson, T.D. (2010). Social Psychology (7th ed.). Upper Saddle River, NJ: Prentice Hall.</ref>
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==Overview==
[[John Dollard|Dollard]] et al. (1939) proposed that aggression was due to [[frustration]], which was described as an unpleasant emotion resulting from any interference with achieving a rewarding goal.<ref>{{Cite book |last1= Dollard |first1=J. | last2= Doob |first2=L.W. | last3= Miller |first3=N.E. | last4= Mowrer |first4=O.H. | last5= Sears |first5=R.R. |title= Frustration and Aggression |publisher= Yale University Press |year=1939 |location= New Haven, CT}}</ref> [[Leonard Berkowitz|Berkowitz]]<ref>{{cite journal | last = Berkowitz | first = L. | year = 1987 | title = Frustrations, appraisals, and aversively stimulated aggression | doi = 10.1037/0033-2909.106.1.59 | journal = Aggressive Behavior | volume = 14 | issue = 1 | pages = 3–11 | pmid = 2667009 }}</ref> extended this [[frustration–aggression hypothesis]] and proposed that it is not so much the frustration as the unpleasant emotion that evokes aggressive tendencies, and that all aversive events produce negative [[Affect (psychology)|affect]] and thereby aggressive tendencies, as well as [[fear]] tendencies. Besides [[Classical conditioning|conditioned]] stimuli, Archer categorized aggression-evoking (as well as fear-evoking) stimuli into three groups; [[pain]], [[novelty]], and [[frustration]], although he also described [[looming]], which refers to an object rapidly moving towards the visual sensors of a subject, and can be categorized as "[[Intensity (physics)|intensity]]."<ref name=":Archer">{{cite book |last=Archer |first=J. |chapter=The organization of aggression and fear in vertebrates |year=1976 |editor1-first=P.P.G. |editor1-last=Bateson |editor2-first=P.H. |editor2-last=Klopfer |title=Perspectives in Ethology (Vol.2) |pages=231–298 |location=New York, NY |publisher=Plenum }}</ref>

Aggression can have adaptive benefits or negative effects. Aggressive behavior is an individual or collective social interaction that is a hostile [[behavior]] with the intention of inflicting damage or harm.<ref name=":3" /><ref name=":4" /> Two broad categories of aggression are commonly distinguished. One includes [[affective]] (emotional) and hostile, reactive, or [[Revenge|retaliatory]] aggression that is a response to provocation, and the other includes instrumental, goal-oriented or [[predatory]], in which aggression is used as a means to achieve a goal.<ref>Berkowitz, L. (1993). [https://www.ncjrs.gov/App/publications/Abstract.aspx?id=142050 Aggression: Its causes, consequences, and control]. New York, NY: McGraw-Hill.</ref> An example of hostile aggression would be a person who punches someone that insulted him or her. An instrumental form of aggression would be [[armed robbery]]. Research on [[violence]] from a range of disciplines lend some support to a distinction between affective and predatory aggression.<ref>{{cite journal |doi=10.1016/j.avb.2003.06.002 |title=Affective and predatory violence: A bimodal classification system of human aggression and violence |journal=Aggression and Violent Behavior |volume=10 |issue=1 |pages=1–30 |year=2004 |last1=McEllistrem |first1=Joseph E. }}</ref> However, some researchers question the usefulness of a hostile versus instrumental distinction in humans, despite its ubiquity in research, because most real-life cases involve mixed motives and interacting causes.<ref>{{cite journal |doi=10.1037/0033-295X.108.1.273 |pmid=11212630 |title=Is it time to pull the plug on hostile versus instrumental aggression dichotomy? |journal=Psychological Review |volume=108 |issue=1 |pages=273–9 |year=2001 |last1=Bushman |first1=Brad J. |last2=Anderson |first2=Craig A. }}</ref>

A number of classifications and dimensions of aggression have been suggested. These depend on such things as whether the aggression is verbal or physical; whether or not it involves [[relational aggression]] such as covert bullying and social manipulation;<ref>Ellie L. Young, David A. Nelson, America B. Hottle, Brittney Warburton, and Bryan K. Young (2010) [http://www.nasponline.org/resources/principals/Relational_Aggression_PLOct10.pdf Relational Aggression Among Students] Principal Leadership, October, copyright the National Association of Secondary School Principals</ref> whether harm to others is intended or not; whether it is carried out actively or expressed passively; and whether the aggression is aimed directly or indirectly. Classification may also encompass aggression-related emotions (e.g., [[anger]]) and mental states (e.g., [[impulsivity]], [[hostility]]).<ref>{{cite journal |doi=10.1016/j.neubiorev.2005.04.015 |pmid=16081158 |title=Aggression, and some related psychological constructs (anger, hostility, and impulsivity) Some comments from a research project |journal=Neuroscience & Biobehavioral Reviews |volume=30 |issue=3 |pages=276–91 |year=2006 |last1=Ramírez |first1=J.M. |last2=Andreu |first2=J.M. }}</ref> Aggression may occur in response to non-social as well as social factors, and can have a close relationship with stress coping style.<ref>{{cite journal |doi=10.1159/000105491 |pmid=17914259 |title=Neurobiological Mechanisms of Aggression and Stress Coping: A Comparative Study in Mouse and Rat Selection Lines |journal=Brain, Behavior and Evolution |volume=70 |issue=4 |pages=274–85 |year=2007 |last1=Veenema |first1=Alexa H. |last2=Neumann |first2=Inga D. }}</ref> Aggression may be [[Threat display|displayed]] in order to [[intimidate]].

The operative definition of aggression may be affected by [[morality|moral]] or [[politics|political]] views. Examples are the axiomatic moral view called the [[non-aggression principle]] and the political rules governing the behavior of one country toward another.<ref>{{cite news|last=Simons|first=Marlise|title=International Court May Define Aggression as Crime|url=https://www.nytimes.com/2010/05/31/world/31icc.html|date=May 2010|work=The New York Times}}</ref> Likewise in competitive [[sports]], or in the [[workplace]], some forms of aggression may be sanctioned and others not (see [[Workplace aggression]]).<ref>Nathaniel Snow [http://bleacherreport.com/articles/367924-violence-and-aggression-in-sports-an-in-depth-look-part-one Violence and Aggression in Sports: An In-Depth Look (Part One)] ([http://bleacherreport.com/articles/368875-violence-and-aggression-in-sports-an-in-depth-look-part-2 Part 2] [http://bleacherreport.com/articles/370563-violence-and-aggression-in-sports-an-in-depth-look-part-3 Part 3]) Bleacher Report, 23 March 2010</ref> Aggressive behaviors are associated with adjustment problems and several psychopathological symptoms such as [[antisocial personality disorder]], [[borderline personality disorder]], and [[intermittent explosive disorder]].<ref name=":5" />

Biological approaches conceptualize aggression as an internal energy released by external stimuli, a product of evolution through natural selection, part of genetics, and a product of hormonal fluctuations. Psychological approaches conceptualize aggression as a destructive instinct, a response to frustration, an affect excited by a negative stimulus, a result of observed learning of society and diversified reinforcement, and a result of variables that affect personal and situational environments.<ref name="Krahé2013">{{cite book|author=Barbara Krahé|title=The Social Psychology of Aggression: 2nd Edition|url=https://books.google.com/books?id=vUg5NGU2lEcC&pg=PT16|date=11 February 2013|publisher=Psychology Press|isbn=978-1-136-17772-9|page=16}}</ref><ref name="Englander2003">{{cite book|author=Elizabeth Kande Englander|title=Understanding Violence|url=https://books.google.com/books?id=AcV5AgAAQBAJ&pg=PA55|date=30 January 2003|publisher=Psychology Press|isbn=978-1-135-65676-8|pages=55–86}}</ref>

==Etymology==
The term aggression comes from the [[Latin]] word ''aggressio'', meaning attack. The Latin was itself a joining of ''ad''- and ''gradi''-, which meant "step at". The first known use dates back to 1611, in the sense of an unprovoked attack.<ref>[http://www.merriam-webster.com/dictionary/aggression Merriam-Webster: Aggression] Retrieved 10 January 2012</ref>

A psychological sense of "hostile or destructive behavior" dates back to a 1912 English translation of [[Sigmund Freud]]'s writing.<ref>[http://www.etymonline.com/index.php?term=aggression Online Etymology Dictionary: Aggression] Retrieved 10 January 2012</ref> [[Alfred Adler]] theorized about an "aggressive drive" in 1908. [[parenting|Child raising]] experts began to refer to aggression, rather than anger, from the 1930s.<ref>Stearns, D. C. (2003). [http://www.faqs.org/childhood/A-Ar/Anger-and-Aggression.html Anger and aggression.] Encyclopedia of Children and Childhood: In History and Society. Paula S. Fass (Ed.). Macmillan Reference Books</ref>

==Ethology==
[[File:Elephant seal fight Part-1.jpg|right|thumb|250px|Male [[elephant seal]]s fighting]]
[[Ethology|Ethologists]] study aggression as it relates to the interaction and [[evolution]] of animals in natural settings. In such settings aggression can involve bodily contact such as biting, hitting or pushing, but most conflicts are settled by threat displays and intimidating thrusts that cause no physical harm. This form of aggression may include the display of body size, antlers, claws or teeth; stereotyped signals including facial expressions; vocalizations such as bird song; the release of chemicals; and changes in coloration.<ref>Van Staaden, M.J, Searcy, W.A. & Hanlon, R.T. 'Signaling Aggression' in [https://books.google.com/books?id=zkp7nMtMNOgC Aggression] Academic Press, Stephen F. Goodwin, 2011</ref> The term [[agonistic behaviour|agonistic behavior]] is sometimes used to refer to these forms of behavior.

Most ethologists believe that aggression confers biological advantages. Aggression may help an animal secure [[territory (animal)|territory]], including resources such as food and water. Aggression between males often occurs to secure mating opportunities, and results in selection of the healthier/more vigorous animal. Aggression may also occur for self-protection or to protect offspring.<ref>{{cite journal |doi=10.1139/z92-150 |title=Functional aspects of maternal aggression in mammals |journal=Canadian Journal of Zoology |volume=70 |issue=6 |pages=1069–77 |year=1992 |last1=Maestripieri |first1=Dario |bibcode=1992CaJZ...70.1069M }}</ref> Aggression between groups of animals may also confer advantage; for example, hostile behavior may force a population of animals into a new territory, where the need to adapt to a new environment may lead to an increase in genetic flexibility.<ref>Psychology- The Science of Behaviour, pg 420, Neil R Clarkson (4th Edition)</ref>

===Between species and groups===
The most apparent type of [[interspecific]] aggression is that observed in the interaction between a [[predator]] and its [[prey]]. However, according to many researchers, [[predation]] is not aggression. A cat does not hiss or arch its back when pursuing a rat, and the active areas in its [[hypothalamus]] resemble those that reflect hunger rather than those that reflect aggression.<ref>Gleitman, Henry, Alan J. Fridlund, and Daniel Reisberg. Psychology. 6th ed. New York: W W Norton and Company, 2004. 431–432.</ref> However, others refer to this behavior as predatory aggression, and point out cases that resemble hostile behavior, such as mouse-killing by rats.<ref>{{cite book |last1=Gendreau |first1=P. L. |last2=Archer |first2=J. |chapter=Subtypes of Aggression in Humans and Animals |pages=25–46 |editor1-last=Tremblay |editor1-first=Richard Ernest |editor2-last=Hartup |editor2-first=Willard W. |editor3-last=Archer |editor3-first=John |title=Developmental Origins of Aggression |date=2005 |publisher=Guilford Press |isbn=978-1-59385-110-1 }}</ref> In [[aggressive mimicry]] a predator has the appearance of a harmless organism or object attractive to the prey; when the prey approaches, the predator attacks.

An animal defending against a predator may engage in either "[[Fight-or-flight response|fight or flight]]" or "[[tend and befriend]]" in response to predator attack or threat of attack, depending on its estimate of the predator's strength relative to its own. Alternative defenses include a range of [[antipredator adaptation]]s, including [[alarm signal]]s. An example of an alarm signal is Nerol, a chemical which is found in the mandibular glands of ''[[Trigona fulviventris]]'' individuals.<ref name=":0">{{cite journal |doi=10.1007/BF00990750 |pmid=24413960 |title=Nerol: An alarm substance of the stingless bee, Trigona fulviventris (Hymenoptera: Apidae) |journal=Journal of Chemical Ecology |volume=8 |issue=9 |pages=1167–81 |year=1982 |last1=Johnson |first1=L. K. |last2=Wiemer |first2=D. F. |bibcode=1982JCEco...8.1167J }}</ref> Release of Nerol by T. fulviventris individuals in the nest has been shown to decrease the number of individuals leaving the nest by fifty percent, as well as increasing aggressive behaviors like biting.<ref name=":0" /> Alarm signals like nerol can also act as attraction signals; in T. fulviventris, individuals that have been captured by a predator may release nerol to attract nest mates, who will proceed to attack or bite the predator.<ref name=":0" />

Aggression between groups is determined partly by willingness to fight, which depends on a number of factors including numerical advantage, distance from home territories, how often the groups encounter each other, competitive abilities, differences in body size, and whose territory is being invaded.<ref>{{cite journal |doi=10.1016/j.jebo.2017.05.004 |title=The logic of animal intergroup conflict: A review |journal=Journal of Economic Behavior and Organization |year=2017 |last1=Rusch |first1=H. |last2=Gavrilets |first2=S.|volume=178 |pages=1014–1030 |doi-access=free }}</ref> Also, an individual is more likely to become aggressive if other aggressive group members are nearby.<ref>{{cite journal |doi=10.1098/rspb.2006.3626 |pmid=17015327 |pmc=1635503 |jstor=25223670 |title=Numerical assessment affects aggression and competitive ability: A team-fighting strategy for the ant Formica xerophila |journal=Proceedings of the Royal Society B: Biological Sciences |volume=273 |issue=1602 |pages=2737–42 |year=2006 |last1=Tanner |first1=C. J }}</ref> One particular phenomenon – the formation of coordinated coalitions that raid neighboring territories to kill [[conspecific]]s – has only been documented in two species in the animal kingdom: [[Common chimpanzee|'common' chimpanzees]] and [[humans]].<ref>{{cite journal |doi=10.1016/j.cub.2010.04.021 |pmid=20620900 |title=Lethal intergroup aggression leads to territorial expansion in wild chimpanzees |journal=Current Biology |volume=20 |issue=12 |pages=R507–8 |year=2010 |last1=Mitani |first1=John C. |last2=Watts |first2=David P. |last3=Amsler |first3=Sylvia J. |doi-access=free |bibcode=2010CBio...20.R507M }}</ref>

===Within a group===
Aggression between conspecifics in a group typically involves access to resources and breeding opportunities. One of its most common functions is to establish a [[dominance hierarchy]]. This occurs in many species by aggressive encounters between contending males when they are first together in a common environment.<ref name="adamsonetal2006">{{cite journal |pmid=10574728 |url=http://jeb.biologists.org/cgi/pmidlookup?view=long&pmid=10574728 |year=1999 |last1=Issa |first1=F. A. |title=Dominance hierarchy formation in juvenile crayfish procambarus clarkii |journal=The Journal of Experimental Biology |volume=202 |pages=3497–506 |last2=Adamson |first2=D. J. |last3=Edwards |first3=D. H. |issue=24|doi = 10.1242/jeb.202.24.3497| bibcode=1999JExpB.202.3497I |url-access=subscription }}</ref> Usually the more aggressive animals become the more dominant.<ref>{{cite journal |doi=10.1016/j.beproc.2006.05.004 |pmid=16815645 |title=Social relationships in a herd of Sorraia horses |journal=Behavioural Processes |volume=73 |issue=2 |pages=170–7 |year=2006 |last1=Heitor |first1=Filipa |last2=Oom |first2=Maria do Mar |last3=Vicente |first3=Luís }}</ref><ref>{{cite journal |doi=10.1086/503445 |pmid=16615035 |title=Individual Variation in Social Aggression and the Probability of Inheritance: Theory and a Field Test |journal=The American Naturalist |volume=167 |issue=6 |pages=837–52 |year=2006 |last1=Cant |first1=Michael A. |last2=Llop |first2=Justine B. |last3=Field |first3=Jeremy |bibcode=2006ANat..167..837C |hdl=10871/26263 |hdl-access=free }}</ref> In test situations, most of the conspecific aggression ceases about 24 hours after the group of animals is brought together.<ref name="adamsonetal2006"/><ref>{{cite journal |pmid=16869278 |year=2006 |last1=Bragin |first1=A. V. |title=The experimental model of establishment and maintenance of social hierarchy in laboratory mice |journal=Zhurnal Vysshei Nervnoi Deiatelnosti imeni I P Pavlova |volume=56 |issue=3 |pages=412–9 |last2=Osadchuk |first2=L. V. |last3=Osadchuk |first3=A. V. }}</ref> Aggression has been defined from this viewpoint as "behavior which is intended to increase the social dominance of the organism relative to the dominance position of other organisms".<ref name="Ferguson & Beaver, 2009">{{cite journal |doi=10.1016/j.avb.2009.03.005 |title=Natural born killers: The genetic origins of extreme violence |journal=Aggression and Violent Behavior |volume=14 |issue=5 |pages=286–94 |year=2009 |last1=Ferguson |first1=Christopher J. |last2=Beaver |first2=Kevin M. }}</ref> Losing confrontations may be called [[social defeat]], and winning or losing is associated with a range of practical and psychological consequences.<ref>{{cite journal |doi=10.1017/S146479310500686X |pmid=16460581 |title=Modulation of aggressive behaviour by fighting experience: Mechanisms and contest outcomes |journal=Biological Reviews |volume=81 |issue=1 |pages=33–74 |year=2005 |last1=Hsu |first1=Yuying |last2=Earley |first2=Ryan L. |last3=Wolf |first3=Larry L. |doi-access=free }}</ref>

Conflicts between animals occur in many contexts, such as between potential mating partners, between parents and offspring, between siblings and between competitors for resources. Group-living animals may dispute over the direction of travel or the allocation of time to joint activities. Various factors limit the escalation of aggression, including communicative displays, conventions, and routines. In addition, following aggressive incidents, various forms of [[conflict resolution]] have been observed in mammalian species, particularly in gregarious primates. These can mitigate or repair possible adverse consequences, especially for the recipient of aggression who may become vulnerable to attacks by other members of a group. Conciliatory acts vary by species and may involve specific gestures or simply more proximity and interaction between the individuals involved. However, conflicts over food are rarely followed by post conflict reunions, even though they are the most frequent type in foraging primates.<ref>{{cite journal |doi=10.1006/anbe.2002.3071 |title=Conflict resolution following aggression in gregarious animals: A predictive framework |journal=Animal Behaviour |volume=64 |issue=3 |pages=325–43 |year=2002 |last1=Aureli |first1=Filippo |last2=Cords |first2=Marina |last3=Van Schaik |first3=Carel P. }}</ref>

Other questions that have been considered in the study of primate aggression, including in humans, is how aggression affects the organization of a group, what costs are incurred by aggression, and why some primates avoid aggressive behavior.<ref>{{cite book |doi=10.1093/oso/9780195071191.001.0001 |title=Aggression and Peacefulness in Humans and Other Primates |date=1992 |isbn=978-0-19-507119-1 |editor-last1=Silverberg |editor-last2=Gray |editor-first1=James |editor-first2=J Patrick }}{{pn|date=July 2025}}</ref> For example, [[Bonobo|bonobo chimpanzee]] groups are known for low levels of aggression within a partially [[matriarchal]] society. [[Captivity (animal)|Captive]] animals including primates may show abnormal levels of social aggression and self-harm that are related to aspects of the physical or social environment; this depends on the species and individual factors such as gender, age and background (e.g., raised wild or captive).<ref>{{cite journal |doi=10.1016/j.neubiorev.2005.05.002 |pmid=16055188 |title=Enrichment and aggression in primates |journal=Neuroscience & Biobehavioral Reviews |volume=30 |issue=3 |pages=413–36 |year=2006 |last1=Honess |first1=P.E. |last2=Marin |first2=C.M. }}</ref>

===Aggression, fear and curiosity===
Within ethology, it has long been recognized that there is a relation between aggression, [[fear]], and [[curiosity]].<ref>{{Cite book|last=Hinde|first=R.A.|title=Animal Behaviour: A Synthesis of Ethology and Comparative Psychology (2nd Ed.)|publisher=McGraw-Hill|year=1970|location=New York, NY}}</ref> A [[cognitive science|cognitive]] approach to this relationship puts aggression in the broader context of [[cognitive dissonance|inconsistency reduction]], and proposes that aggressive behavior is caused by an inconsistency between a desired, or expected, situation and the actually perceived situation (e.g., "[[Frustration–aggression hypothesis|frustration]]"), and functions to forcefully [[Manipulation (psychology)|manipulate]] the perception into matching the expected situation.<ref>{{Cite book|last=Hebb|first=D.O.|title=The Organisation of Behavior|publisher=Wiley|year=1949|location=New York, NY}}</ref><ref name=":Archer"/><ref name=":Kampen">{{cite journal |last1=van Kampen |first1=Hendrik S. |title=The principle of consistency and the cause and function of behaviour |journal=Behavioural Processes |date=February 2019 |volume=159 |pages=42–54 |doi=10.1016/j.beproc.2018.12.013 |pmid=30562561 }}</ref> In this approach, when the inconsistency between perception and expectancy is small, learning as a result of curiosity reduces inconsistency by updating expectancy to match perception. If the inconsistency is larger, fear or aggressive behavior may be employed to alter the perception in order to make it match expectancy, depending on the size of the inconsistency as well as the specific context. Uninhibited fear results in fleeing, thereby removing the inconsistent stimulus from the perceptual field and resolving the inconsistency. In some cases, thwarted escape may trigger aggressive behavior in an attempt to remove the thwarting stimulus.<ref name=":Kampen"/>

==Evolutionary explanations==
Like many behaviors, aggression can be examined in terms of its ability to help an animal itself survive and reproduce, or alternatively to risk survival and reproduction. This [[cost–benefit analysis]] can be looked at in terms of [[evolution]]. However, there are profound differences in the extent of acceptance of a biological or evolutionary basis for human aggression.<ref>{{cite journal |doi=10.1177/0022002790034003008 |jstor=174228 |title=Humans, Chimps, and Bonobos: The Biological Bases of Aggression, War, and Peacemaking |journal=Journal of Conflict Resolution |volume=34 |issue=3 |pages=553–82 |year=1990 |last1=Somit |first1=A. }}</ref>

===Violence and conflict===
Aggression can involve [[violence]] that may be [[Adaptation|adaptive]] under certain circumstances in terms of [[natural selection]]. This is most obviously the case in terms of attacking prey to obtain food, or in anti-predatory defense. It may also be the case in competition between members of the same species or subgroup, if the average reward (e.g., status, access to resources, protection of self or kin) outweighs average costs (e.g., injury, exclusion from the group, death). There are some hypotheses of specific adaptions for violence in humans under certain circumstances, including for [[homicide]], but it is often unclear what behaviors may have been selected for and what may have been a byproduct, as in the case of collective violence.<ref>{{cite book |last1=Buss |first1=David M. |title=The Murderer Next Door: Why the Mind Is Designed to Kill |date=2006 |publisher=Penguin |isbn=978-1-101-11769-9 }}{{pn|date=July 2025}}</ref><ref>{{cite journal |doi=10.1016/j.avb.2007.04.001 |title=Examining the evidence from small-scale societies and early prehistory and implications for modern theories of aggression and violence |journal=Aggression and Violent Behavior |volume=13 |issue=1 |pages=1–9 |year=2008 |last1=McCall |first1=Grant S. |last2=Shields |first2=Nancy }}</ref><ref>{{cite book |last1=Buss |first1=D. M. |last2=Duntley |first2=J. D. |chapter-url=http://psycnet.apa.org/psycinfo/2006-20303-012 |chapter=The evolution of aggression |year=2006 |editor1-first=M. |editor1-last=Schaller |editor2-first=J. A. |editor2-last=Simpson |editor3-first=D. T. |editor3-last=Kenrick |title=Evolution and Social Psychology |pages=263–86 |location=New York |publisher=Psychology Press }}</ref><ref>{{cite journal |doi=10.1016/j.avb.2011.04.014 |title=Collective violence: An evolutionary perspective |journal=Aggression and Violent Behavior |volume=16 |issue=5 |pages=428–36 |year=2011 |last1=Durrant |first1=Russil }}</ref>

Although aggressive encounters are ubiquitous in the animal kingdom, with often high stakes, most encounters that involve aggression may be resolved through posturing, or displaying and trial of strength. [[Game theory]] is used to understand how such behaviors might spread by [[natural selection]] within a population, and potentially become 'Evolutionary Stable Strategies'. An initial model of resolution of conflicts is the [[hawk-dove game]]. Others include the [[Risk management|Sequential assessment model]] and the [[Attrition warfare|Energetic war of attrition]]. These try to understand not just one-off encounters but protracted stand-offs, and mainly differ in the criteria by which an individual decides to give up rather than risk loss and harm in physical conflict (such as through estimates of [[resource holding potential]]).<ref>{{cite journal |last1=Briffa |first1=Mark |year=2010 |url=http://www.nature.com/scitable/knowledge/library/territoriality–and–aggression–13240908 |title=Territoriality and Aggression |journal=Nature Education Knowledge |volume=3 |issue=10 |pages=81 }}</ref>

===Gender===
{{Sex differences}}

====General====
Gender plays an important role in human aggression. There are multiple theories that seek to explain findings that males and females of the same species can have differing aggressive behaviors. One review concluded that male aggression tended to produce pain or physical injury, whereas female aggression tended towards psychological or social harm.<ref name=pmid3797558>{{cite journal |doi=10.1037/0033-2909.100.3.309 |pmid=3797558 |title=Gender and aggressive behavior: A meta-analytic review of the social psychological literature |journal=Psychological Bulletin |volume=100 |issue=3 |pages=309–30 |year=1986 |last1=Eagly |first1=Alice H. |last2=Steffen |first2=Valerie J. }}</ref>

In general, [[sexual dimorphism]] can be attributed to greater [[intraspecific competition]] in one sex, either between rivals for access to mates and/or to be [[mate choice|chosen by mates]]. This may stem from the other gender being constrained by providing greater [[parental investment]], in terms of factors such as [[gamete]] production, [[gestation]], [[lactation]], or upbringing of young. Although there is much variation in species, generally the more physically aggressive sex is the male, particularly in mammals. In species where parental care by both sexes is required, there tends to be less of a difference. When the female can leave the male to care for the offspring, then females may be the larger and more physically aggressive. Competitiveness despite parental investment has also been observed in some species.<ref>{{cite journal |doi=10.1038/nature05386 |pmid=17183322 |title=Intrasexual competition and sexual selection in cooperative mammals |journal=Nature |volume=444 |issue=7122 |pages=1065–8 |year=2006 |last1=Clutton-Brock |first1=T. H. |last2=Hodge |first2=S. J. |last3=Spong |first3=G. |last4=Russell |first4=A. F. |last5=Jordan |first5=N. R. |last6=Bennett |first6=N. C. |last7=Sharpe |first7=L. L. |last8=Manser |first8=M. B. |bibcode=2006Natur.444.1065C }}</ref> A related factor is the rate at which males and females are able to mate again after producing offspring, and the basic principles of [[sexual selection]] are also influenced by ecological factors affecting the ways or extent to which one sex can compete for the other. The role of such factors in human evolution is controversial.

The pattern of male and female aggression is argued to be consistent with evolved sexually selected behavioral differences, while alternative or complementary views emphasize conventional [[gender role|social roles]] stemming from physical evolved differences.<ref>{{cite journal |last1=Archer |first1=John |title=Does sexual selection explain human sex differences in aggression? |journal=Behavioral and Brain Sciences |date=August 2009 |volume=32 |issue=3–4 |pages=249–266 |doi=10.1017/S0140525X09990951 |pmid=19691899 }}</ref> Aggression in women may have evolved to be, on average, less physically dangerous and more covert or [[Indirect aggression|indirect]].<ref>{{cite journal |doi=10.1017/s0140525x99001818 |title=Staying alive: Evolution, culture, and women's intrasexual aggression |journal=Behavioral and Brain Sciences |volume=22 |issue=2 |year=1999 |last1=Campbell |first1=Anne |pmid=11301523 |pages=203–14; discussion 214–52}}</ref><ref>[https://books.google.com/books?id=esDW3xTKoLIC The Handbook of Evolutionary Psychology], edited by David M. Buss, John Wiley & Sons, Inc., 2005. Chapter 21 by Anne Campbell.{{page needed|date=February 2017}}</ref> However, there are critiques of using animal behavior to explain human behavior, especially in the application of evolutionary explanations to contemporary human behavior, including differences between genders.<ref>Zuk, M. "[https://books.google.com/books/about/Sexual_selections.html?id=-Sjsmhm82wUC Sexual Selections: What We Can and Can't Learn about Sex from Animals.]" University of California Press, 2002{{page needed|date=February 2017}}</ref>

According to the 2015 ''[[International Encyclopedia of the Social & Behavioral Sciences]]'', sex differences in aggression is one of the most robust and oldest findings in psychology.<ref name = "274956064 Gender diff">{{cite book |doi=10.1016/B978-0-08-097086-8.25100-3 |chapter=Gender Differences in Personality and Social Behavior |title=International Encyclopedia of the Social & Behavioral Sciences |pages=750–6 |year=2015 |last1=Del Giudice |first1=Marco |hdl=2318/1852940 |isbn=978-0-08-097087-5 }}</ref> Past meta-analyses in the encyclopedia found males regardless of age engaged in more physical and verbal aggression while small effect for females engaging in more indirect aggression such as rumor spreading or gossiping.<ref name= "274956064 Gender diff"/> It also found males tend to engage in more unprovoked aggression at higher frequency than females.<ref name= "274956064 Gender diff"/> This analysis also conforms with the ''Oxford Handbook of Evolutionary Psychology'' which reviewed past analysis which found men to use more verbal and physical aggression with the difference being greater in the physical type.<ref>{{cite book |doi=10.1093/oxfordhb/9780198568308.013.0025 |year=2012 |last1=Campbell |first1=Anne |chapter=Sex differences in aggression |chapter-url={{Google books|8K7Hc09xcQAC|page=365|plainurl=yes}} |pages=365–82 |editor1-first=Louise |editor1-last=Barrett |editor2-first=Robin |editor2-last=Dunbar |title=Oxford Handbook of Evolutionary Psychology |isbn=978-0-19-856830-8 }}</ref>

There are more recent findings that show that differences in male and female aggression appear at about two years of age, though the differences in aggression are more consistent in middle-aged children and adolescence. Tremblay, Japel and Pérusse (1999) asserted that physically aggressive behaviors such as kicking, biting and hitting are age-typical expressions of innate and spontaneous reactions to biological drives such as anger, hunger, and affiliation.<ref>{{cite journal |doi=10.1002/bsl.2035 |pmid=22996132 |title=Gender Differences in Physical Aggression and Associated Developmental Correlates in a Sample of Canadian Preschoolers |journal=Behavioral Sciences & the Law |volume=30 |issue=5 |pages=643–71 |year=2012 |last1=Lussier |first1=Patrick |last2=Corrado |first2=Raymond |last3=Tzoumakis |first3=Stacy }}</ref> Girls' [[relational aggression]], meaning non-physical or indirect, tends to increase after age two while physical aggression decreases. There was no significant difference in aggression between males and females before two years of age.<ref name="Landsford 2012 298–308">{{cite journal |doi=10.1002/ab.21433 |pmid=23935227 |pmc=3736589 |title=Boys' and Girls' Relational and Physical Aggression in Nine Countries |journal=Aggressive Behavior |volume=38 |issue=4 |pages=298–308 |year=2012 |last1=Lansford |first1=Jennifer E. |last2=Skinner |first2=Ann T. |last3=Sorbring |first3=Emma |last4=Giunta |first4=Laura Di |last5=Deater-Deckard |first5=Kirby |last6=Dodge |first6=Kenneth A. |last7=Malone |first7=Patrick S. |last8=Oburu |first8=Paul |last9=Pastorelli |first9=Concetta |last10=Tapanya |first10=Sombat |last11=Uribe Tirado |first11=Liliana Maria |last12=Zelli |first12=Arnaldo |last13=Al-Hassan |first13=Suha M. |last14=Peña Alampay |first14=Liane |last15=Bacchini |first15=Dario |last16=Bombi |first16=Anna Silvia |last17=Bornstein |first17=Marc H. |last18=Chang |first18=Lei }}</ref> A possible explanation for this could be that girls develop language skills more quickly than boys, and therefore have better ways of verbalizing their wants and needs. They are more likely to use communication when trying to retrieve a toy with the words "Ask nicely" or "Say please."<ref name="Hay 2011 158–75">{{cite journal |doi=10.1111/j.2044-835x.2011.02028.x |pmid=21592146 |title=The emergence of gender differences in physical aggression in the context of conflict between young peers |journal=British Journal of Developmental Psychology |volume=29 |issue=2 |pages=158–75 |year=2011 |last1=Hay |first1=Dale F. |last2=Nash |first2=Alison |last3=Caplan |first3=Marlene |last4=Swartzentruber |first4=Jan |last5=Ishikawa |first5=Fumiko |last6=Vespo |first6=Jo Ellen }}</ref>

According to the journal of ''Aggressive Behaviour'', an analysis across 9 countries found boys reported more in the use of physical aggression.<ref name="Landsford 2012 298–308"/> At the same time no consistent sex differences emerged within relational aggression.<ref name="Landsford 2012 298–308" /> It has been found that girls are more likely than boys to use reactive aggression and then retract, but boys are more likely to increase rather than to retract their aggression after their first reaction. Studies show girls' aggressive tactics included [[gossip]], [[Social rejection|ostracism]], breaking confidences, and criticism of a victim's clothing, appearance, or personality, whereas boys engage in aggression that involves a direct physical and/or verbal assault.<ref>{{cite journal |doi=10.1016/j.evolhumbehav.2005.11.001 |title=Sex differences in indirect aggression |journal=Evolution and Human Behavior |volume=27 |issue=3 |pages=231–45 |year=2006 |last1=Hess |first1=Nicole H. |last2=Hagen |first2=Edward H. }}</ref> This could be due to the fact that girls' frontal lobes develop earlier than boys, allowing them to self-restrain.<ref name="Hay 2011 158–75"/>

One factor that shows insignificant differences between male and female aggression is in sports. In sports, the rate of aggression in both contact and non-contact sports is relatively equal. Since the establishment of Title IX, female sports have increased in competitiveness and importance, which could contribute to the evening of aggression and the "need to win" attitude between both genders. Among sex differences found in adult sports were that females have a higher scale of indirect hostility while men have a higher scale of assault.<ref>{{cite journal |last=Keeler |first=Linda A |title=The differences in sport aggression, life aggression, and life assertion among adult male and female collision, contact, and non-contact sport athletes |journal=Journal of Sport Behavior |year=2007 |volume=30 |issue=1 |pages=57–76 |url=https://www.proquest.com/openview/19a147ae672eead5e95f7141e6116794/1 }}</ref> Another difference found is that men have up to 20 times higher levels of [[testosterone]] than women.

==== In intimate relationships ====
Some studies suggest that romantic involvement in adolescence decreases aggression in males and females, but decreases at a higher rate in females. Females will seem more desirable to their mate if they fit in with society and females that are aggressive do not usually fit well in society. They can often be viewed as antisocial. Female aggression is not considered the norm in society and going against the norm can sometimes prevent one from getting a mate.<ref>{{cite journal |doi=10.1002/ab.20404 |pmid=21748751 |pmc=4332584 |title=Developmental trajectories of aggression from late childhood through adolescence: Similarities and differences across gender |journal=Aggressive Behavior |volume=37 |issue=5 |pages=387–404 |year=2011 |last1=Xie |first1=Hongling |last2=Drabick |first2=Deborah A.G. |last3=Chen |first3=Diane }}</ref> However, studies have shown that an increasing number of women are getting arrested on [[domestic violence]] charges. In many states, women now account for a quarter to a third of all domestic violence arrests, up from less than 10 percent a decade ago.

The new statistics reflect a reality documented in research: women are perpetrators as well as victims of family violence.<ref>{{cite news|last=Young|first=Cathy|title=Feminists Play the Victim Game|url=https://www.nytimes.com/1999/11/26/opinion/feminists-play-the-victim-game.html|work=The New York Times|access-date=6 December 2012|date=26 November 1999}}</ref> However, another equally possible explanation is a case of improved diagnostics: it has become more acceptable for men to report female domestic violence to the authorities while at the same time actual female domestic violence has not increased at all.<ref>{{Cite web|title=What To Do If Your Partner Has A Steamroller Personality|url=https://newvisionpsychology.com.au/couples-counselling-sydney/partner-has-steamroller-personality/|url-status=live|access-date=|website=|language=|archive-url=https://web.archive.org/web/20210907064134/https://newvisionpsychology.com.au/couples-counselling-sydney/partner-has-steamroller-personality/ |archive-date=7 September 2021 }}</ref> This could be the case in a situation where men had become less ashamed of reporting [[Domestic violence against men|female violence against them]]⁠{{nowrap|{{px2}}{{mdash}}{{px2}}}}such a situation could conceivably lead to an increasing number of women being arrested despite the actual number of violent women remaining the same.

In addition, males in competitive sports are often advised by their coaches not to be in intimate relationships based on the premise that they become more docile and less aggressive during an athletic event. The circumstances in which males and females experience aggression are also different. A study showed that social anxiety and stress were positively correlated with aggression in males, meaning that as stress and social anxiety increase, so does aggression. Furthermore, a male with higher social skills has a lower rate of aggressive behavior than a male with lower social skills. In females, higher rates of aggression were only correlated with higher rates of stress. Other than biological factors that contribute to aggression, there are physical factors as well.<ref>{{cite journal |last1=Al-Ali |first1=Majed. M. |last2=Singh |first2=Ajai Pratap |last3=Smekal |first3=Vladimir |title=Social anxiety in relation to social skills, aggression, and stress among male and female commercial institute students |journal=Education |year=2011 |volume=132 |issue=2 |pages=351–61 |url=https://www.questia.com/library/journal/1G1-278759334/social-anxiety-in-relation-to-social-skills-aggression }}</ref>

==== Physiological factors ====
Regarding sexual dimorphism, humans fall into an intermediate group with moderate sex differences in body size but relatively large [[testes]]. This is a typical pattern of primates where several males and females live together in a group and the male faces an intermediate number of challenges from other males compared to exclusive [[polygyny]] and [[monogamy]] but frequent [[sperm competition]].<ref name=Oxford2007>{{cite book |doi=10.1093/oxfordhb/9780198568308.013.0030 |year=2012 |last1=Low |first1=Bobbi S. |chapter=Ecological and socio-cultural impacts on mating and marriage systems |chapter-url={{Google books|8K7Hc09xcQAC|page=449|plainurl=yes}} |pages=449–62 |editor1-first=Louise |editor1-last=Barrett |editor2-first=Robin |editor2-last=Dunbar |title=Oxford Handbook of Evolutionary Psychology |isbn=978-0-19-856830-8 }}</ref>

[[Evolutionary psychology]] and [[sociobiology]] have also discussed and produced theories for some specific forms of male aggression such as [[sociobiological theories of rape]] and theories regarding the [[Cinderella effect]].<ref name=":1">{{cite journal |doi=10.1098/rstb.2011.0301 |pmid=22271783 |pmc=3260849 |jstor=41433544 |title=Evolution and the psychology of intergroup conflict: The male warrior hypothesis |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=367 |issue=1589 |pages=670–9 |year=2012 |last1=McDonald |first1=M. M. |last2=Navarrete |first2=C. D. |last3=Van Vugt |first3=M. }}</ref><ref name=":2">{{cite journal |last1=Vugt |first1=Mark Van |last2=Cremer |first2=David De |last3=Janssen |first3=Dirk P. |title=Gender Differences in Cooperation and Competition: The Male-Warrior Hypothesis |journal=Psychological Science |date=January 2007 |volume=18 |issue=1 |pages=19–23 |doi=10.1111/j.1467-9280.2007.01842.x |pmid=17362372 }}</ref>

==Physiology==

===Brain pathways===
Many researchers focus on the brain to explain aggression. Numerous circuits within both neocortical and subcortical structures play a central role in controlling aggressive behavior, depending on the species, and the exact role of pathways may vary depending on the type of trigger or intention.<ref name=":6" /><ref name=":4" />

In mammals, the [[hypothalamus]] and [[periaqueductal gray]] of the [[midbrain]] are critical areas, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of both behavioral and [[autonomic nervous system|autonomic]] components of aggression in these species, including vocalization. Electrical stimulation of the hypothalamus causes aggressive behavior<ref>{{cite journal |doi=10.1016/0006-8993(83)90764-3 |pmid=6681724 |title=Discriminant analysis of the localization of aggression-inducing electrode placements in the hypothalamus of male rats |journal=Brain Research |volume=260 |issue=1 |pages=61–79 |year=1983 |last1=Kruk |first1=Menno R. |last2=Van Der Poel |first2=A.M. |last3=Meelis |first3=W. |last4=Hermans |first4=J. |last5=Mostert |first5=P.G. |last6=Mos |first6=J. |last7=Lohman |first7=A.H.M. }}</ref> and the hypothalamus has receptors that help determine aggression levels based on their interactions with serotonin and vasopressin.<ref>{{cite journal |pmid=9151749 |pmc=6573530 |year=1997 |last1=Ferris |first1=C. F. |title=Vasopressin/serotonin interactions in the anterior hypothalamus control aggressive behavior in golden hamsters |journal=The Journal of Neuroscience |volume=17 |issue=11 |pages=4331–40 |last2=Melloni |first2=R. H. Jr. |last3=Koppel |first3=G |last4=Perry |first4=K. W. |last5=Fuller |first5=R. W. |last6=Delville |first6=Y |doi=10.1523/JNEUROSCI.17-11-04331.1997 }}</ref> In rodents, activation of [[estrogen receptor]]-expressing neurons in the ventrolateral portion of the [[ventromedial hypothalamus]] (VMHvl) was found to be sufficient to initiate aggression in both males and females.<ref>{{cite journal |last1=Lee |first1=H |title=Scalable control of mounting and attack by Esr1+ neurons in the ventromedial hypothalamus |journal=Nature |date=2014 |volume=509 |issue=7502 |pages=627–32 |doi=10.1038/nature13169 |pmid=24739975 |pmc=4098836 |bibcode=2014Natur.509..627L }}</ref><ref>{{cite journal |last1=Hashikawa |first1=K |title=Esr1+ cells in the ventromedial hypothalamus control female aggression |journal=Nat. Neurosci. |date=2017 |volume=11 |issue=20 |pages=1580–1590 |doi=10.1038/nn.4644 |pmid=28920934 |pmc=5953764 }}</ref> Midbrain areas involved in aggression have direct connections with both the [[brainstem]] nuclei controlling these functions, and with structures such as the [[amygdala]] and [[prefrontal cortex]].

Stimulation of the amygdala results in augmented aggressive behavior in hamsters,<ref>{{cite journal |doi=10.1037/0735-7044.110.2.401 |pmid=8731066 |title=Brief, high-frequency stimulation of the corticomedial amygdala induces a delayed and prolonged increase of aggressiveness in male Syrian golden hamsters |journal=Behavioral Neuroscience |volume=110 |issue=2 |pages=401–12 |year=1996 |last1=Potegal |first1=M. |last2=Hebert |first2=M. |last3=Decoster |first3=M. |last4=Meyerhoff |first4=J. L. }}</ref><ref name="Potegal, M 1996. pp. 869-880"/> while [[lesions]] of an [[homology (biology)|evolutionarily homologous]] area in the lizard greatly reduce competitive drive and aggression (Bauman et al. 2006).<ref>{{cite journal |doi=10.1016/0031-9384(84)90088-X |pmid=6538977 |title=Role of the amygdala in the reproductive and aggressive behavior of the lizard, Anolis carolinensis |journal=Physiology & Behavior |volume=32 |issue=1 |pages=147–51 |year=1984 |last1=Greenberg |first1=Neil |last2=Scott |first2=Michelle |last3=Crews |first3=David }}</ref> In [[rhesus monkeys]], neonatal lesions in the amygdala or hippocampus results in reduced expression of social dominance, related to the regulation of aggression and fear.<ref>{{cite journal |doi=10.1037/0735-7044.120.4.749 |pmid=16893283 |title=The expression of social dominance following neonatal lesions of the amygdala or hippocampus in rhesus monkeys (Macaca mulatta) |journal=Behavioral Neuroscience |volume=120 |issue=4 |pages=749–60 |year=2006 |last1=Bauman |first1=M. D. |last2=Toscano |first2=J. E. |last3=Mason |first3=W. A. |last4=Lavenex |first4=P. |last5=Amaral |first5=D. G. |url=http://doc.rero.ch/record/6372/files/lavenex_esd.pdf }}</ref> Several experiments in attack-primed Syrian golden hamsters, for example, support the claim of circuitry within the amygdala being involved in control of aggression.<ref name="Potegal, M 1996. pp. 869-880">{{cite journal |doi=10.1016/0306-4522(96)00236-9 |pmid=8951880 |title=Attack priming in female Syrian golden hamsters is associated with a c-fos-coupled process within the corticomedial amygdala |journal=Neuroscience |volume=75 |issue=3 |pages=869–80 |year=1996 |last1=Potegal |first1=M. |last2=Ferris |first2=C.F. |last3=Hebert |first3=M. |last4=Meyerhoff |first4=J. |last5=Skaredoff |first5=L. |url=https://zenodo.org/record/1258481 }}</ref> The role of the amygdala is less clear in primates and appears to depend more on situational context, with lesions leading to increases in either social affiliatory or aggressive responses. [[Amygdalotomy]], which involves removing or destroying parts of the amygdala, has been performed on people to reduce their violent behaviour.

The broad area of the cortex known as the [[prefrontal cortex]] (PFC) is crucial for [[self-control]] and inhibition of impulses, including inhibition of aggression and emotions. Reduced activity of the prefrontal cortex, in particular its medial and [[Orbitofrontal cortex|orbitofrontal]] portions, has been associated with violent/antisocial aggression.<ref>Paus, T. 'Mapping Brain Development' in [https://books.google.com/books/about/Developmental_origins_of_aggression.html?id=XmSfJEl2v4sC Developmental Origins of Aggression], 2005, The Guilford Press.</ref> In addition, reduced [[response inhibition]] has been found in violent offenders, compared to non-violent offenders.<ref name=":6">{{cite journal |last1=Meijers |first1=J. |last2=Harte |first2=J. M. |last3=Meynen |first3=G. |last4=Cuijpers |first4=P. |title=Differences in executive functioning between violent and non-violent offenders |journal=Psychological Medicine |date=July 2017 |volume=47 |issue=10 |pages=1784–1793 |doi=10.1017/S0033291717000241 |pmid=28173890 |hdl=1871.1/371bdff2-a6df-45ee-a1fd-90995ea811d7 |hdl-access=free }}</ref>

The role of the chemicals in the brain, particularly [[neurotransmitters]], in aggression has also been examined. This varies depending on the pathway, the context and other factors such as gender. A deficit in [[serotonin]] has been theorized to have a primary role in causing impulsivity and aggression. At least one epigenetic study supports this supposition.<ref name=pmid27720744>{{cite journal |doi=10.1016/j.bbr.2016.10.009 |pmid=27720744 |title=The association of serotonin receptor 3A methylation with maternal violence exposure, neural activity, and child aggression |journal=Behavioural Brain Research |volume=325 |issue=Pt B |pages=268–277 |year=2016 |last1=Schechter |first1=Daniel S. |last2=Moser |first2=Dominik A. |last3=Pointet |first3=Virginie C. |last4=Aue |first4=Tatjana |last5=Stenz |first5=Ludwig |last6=Paoloni-Giacobino |first6=Ariane |last7=Adouan |first7=Wafae |last8=Manini |first8=Aurélia |last9=Suardi |first9=Francesca |last10=Vital |first10=Marylene |last11=Sancho Rossignol |first11=Ana |last12=Cordero |first12=Maria I. |last13=Rothenberg |first13=Molly |last14=Ansermet |first14=François |last15=Rusconi Serpa |first15=Sandra |last16=Dayer |first16=Alexandre G. |doi-access=free }}</ref> Nevertheless, low levels of serotonin transmission may explain a vulnerability to impulsiveness, potential aggression, and may have an effect through interactions with other neurochemical systems. These include [[dopamine]] systems which are generally associated with attention and motivation toward rewards, and operate at various levels. [[Norepinephrine]], also known as noradrenaline, may influence aggression responses both directly and indirectly through the hormonal system, the [[sympathetic nervous system]] or the [[central nervous system]] (including the brain). It appears to have different effects depending on the type of triggering stimulus, for example social isolation/rank versus shock/chemical agitation which appears not to have a linear relationship with aggression. Similarly, [[GABA]], although associated with inhibitory functions at many CNS synapses, sometimes shows a positive correlation with aggression, including when potentiated by alcohol.<ref>{{cite journal |doi=10.1016/j.bbr.2008.01.003 |pmid=18281105 |title=Development of violence in mice through repeated victory along with changes in prefrontal cortex neurochemistry |journal=Behavioural Brain Research |volume=189 |issue=2 |pages=263–72 |year=2008 |last1=Caramaschi |first1=Doretta |last2=De Boer |first2=Sietse F. |last3=De Vries |first3=Han |last4=Koolhaas |first4=Jaap M. |hdl=11370/f683911d-3c8b-4ba6-a6bb-12cbaf11dead |hdl-access=free }}</ref><ref>Pihl, RO & Benkelfat, C. 'Neuromodulators in the Development and Expression of Inhibition and Aggression' in [https://books.google.com/books/about Developmental_origins_of_aggression.html?id=XmSfJEl2v4sC Developmental Origins of Aggression], 2005, The Guilford Press.</ref>

The hormonal [[neuropeptides]] [[vasopressin]] and [[oxytocin]] play a key role in complex social behaviours in many mammals such as regulating attachment, social recognition, and aggression. Vasopressin has been implicated in male-typical social behaviors which includes aggression. Oxytocin may have a particular role in regulating female bonds with offspring and mates, including the use of protective aggression. Initial studies in humans suggest some similar effects.<ref>{{cite book |doi=10.1016/S0079-6123(08)00428-7 |pmid=18655894 |chapter=Neuropeptides and social behaviour: Effects of oxytocin and vasopressin in humans |title=Advances in Vasopressin and Oxytocin – from Genes to Behaviour to Disease |volume=170 |pages=337–50 |series=Progress in Brain Research |year=2008 |last1=Heinrichs |first1=M |last2=Domes |first2=G |isbn=978-0-444-53201-5 }}</ref><ref>{{cite journal |doi=10.1016/j.biopsycho.2007.09.001 |pmid=17931766 |title=Attachment, aggression and affiliation: The role of oxytocin in female social behavior |journal=Biological Psychology |volume=77 |issue=1 |pages=1–10 |year=2008 |last1=Campbell |first1=Anne |url=https://durham-repository.worktribe.com/output/1496292 }}</ref>

In human, [[aggressive behavior]] has been associated with abnormalities in three principal regulatory systems in the body [[serotonin|serotonin systems]], [[catecholamine|catecholamine systems]], and the [[hypothalamic–pituitary–adrenal axis]]. Abnormalities in these systems also are known to be induced by [[Stress (biology)|stress]], either severe, acute stress or chronic low-grade stress<ref>{{cite journal |doi=10.1300/J076v36n01_04 |title=Effects of the ''Transcendental Meditation'' Program on Neuroendocrine Abnormalities Associated with Aggression and Crime |journal=Journal of Offender Rehabilitation |volume=36 |issue=1–4 |pages=67–87 |year=2003 |last1=Walton |first1=Kenneth G. |last2=Levitsky |first2=Debra K. }}</ref>

===Testosterone===
{{See also|Testosterone#Aggression}}
Early androgenization has an organizational effect on the developing brains of both males and females, making more neural circuits that control sexual behavior as well as intermale and interfemale aggression become more sensitive to testosterone.<ref name="Carlson, N. 2013">{{cite book |last1=Carlson |first1=Neil R |chapter=Hormonal Control of Aggressive Behavior |title=Physiology of Behavior |date=2013 |publisher=Pearson |isbn=978-0-205-23939-9 }}</ref> There are noticeable sex differences in aggression. Testosterone is present to a lesser extent in females, who may be more sensitive to its effects. Animal studies have also indicated a link between incidents of aggression and the individual level of circulating testosterone. However, results in relation to primates, particularly humans, are less clear cut and are at best only suggestive of a positive association in some contexts.<ref name="van goozen">{{cite book |last1=Van Goozen |first1=S |chapter=Hormones and the Developmental Origins of Aggression |editor1-last=Tremblay |editor1-first=Richard Ernest |editor2-last=Hartup |editor2-first=Willard W. |editor3-last=Archer |editor3-first=John |title=Developmental Origins of Aggression |date=2005 |publisher=Guilford Press |isbn=978-1-59385-110-1 }}</ref>

In humans, there is a seasonal variation in aggression associated with changes in testosterone.<ref name="Archer 319–345" /> For example, in some primate species, such as rhesus monkeys and baboons, females are more likely to engage in fights around the time of ovulation as well as right before menstruation.<ref name="Carlson, N. 2013"/> If the results were the same in humans as they are in rhesus monkeys and baboons, then the increase in aggressive behaviors during ovulation is explained by the decline in estrogen levels. This makes normal testosterone levels more effective.<ref>"[http://www.babymed.com/ovulation/3-important-physical-ovulation-symptoms,2001-2015 Three Important Physical Ovulation Symptoms]" from BabyMed.com</ref> Castrated mice and rats exhibit lower levels of aggression. Males castrated as neonates exhibit low levels of aggression even when given testosterone throughout their development.

====Challenge hypothesis====
The [[challenge hypothesis]] outlines the dynamic relationship between plasma testosterone levels and aggression in mating contexts in many species. It proposes that testosterone is linked to aggression when it is beneficial for reproduction, such as in mate guarding and preventing the encroachment of intrasexual rivals. The challenge hypothesis predicts that seasonal patterns in testosterone levels in a species are a function of mating system (monogamy versus polygyny), paternal care, and male-male aggression in [[seasonal breeder]]s.

This pattern between testosterone and aggression was first observed in seasonally breeding birds, such as the [[song sparrow]], where testosterone levels rise modestly with the onset of the breeding season to support basic reproductive functions.<ref name="Wingfield, John C., Ball, Gregory F., Dufty Jr, Alfred M., Hegner, Robert E., Ramenofsky, Marilyn 1987 602–608">{{cite journal |bibcode=1987AmSci..75..602W |title=Testosterone and Aggression in Birds |journal=[[American Scientist]] |volume=75 |issue=6 |pages=602–8 |last1=Wingfield |first1=John C. |last2=Ball |first2=Gregory F. |last3=Dufty |first3=Alfred M. |last4=Hegner |first4=Robert E. |last5=Ramenofsky |first5=Marilyn |year=1987 }}</ref> The hypothesis has been subsequently expanded and modified to predict relationships between testosterone and aggression in other species. For example, chimpanzees, which are continuous breeders, show significantly raised testosterone levels and aggressive male-male interactions when receptive and fertile females are present.<ref>{{cite journal |doi=10.1016/j.anbehav.2003.03.013 |title=Dominance, aggression and testosterone in wild chimpanzees: A test of the 'challenge hypothesis' |journal=[[Animal Behaviour (journal)|Animal Behaviour]] |volume=67 |pages=113–23 |year=2004 |last1=Muller |first1=Martin N |last2=Wrangham |first2=Richard W }}</ref> Currently, no research has specified a relationship between the modified challenge hypothesis and human behavior, or the human nature of [[concealed ovulation]], although some suggest it may apply.<ref name="Archer 319–345">{{cite journal |doi=10.1016/j.neubiorev.2004.12.007 |title=Testosterone and human aggression: An evaluation of the challenge hypothesis |journal=[[Neuroscience & Biobehavioral Reviews]] |volume=30 |issue=3 |pages=319–45 |year=2006 |last1=Archer |first1=John |pmid=16483890 }}</ref>

====Effects on the nervous system====
[[File:Testosterone estradiol conversion.png|thumb|left|Testosterone to Estradiol conversion]]
Another line of research has focused on the proximate effects of circulating testosterone on the nervous system, as mediated by local metabolism within the brain. Testosterone can be metabolized to [[estradiol]] by the enzyme [[aromatase]], or to [[dihydrotestosterone]] (DHT) by [[5α-reductase]].<ref name="Soma2008" />

Aromatase is highly expressed in regions involved in the regulation of aggressive behavior, such as the amygdala and hypothalamus. In studies using genetic knockout techniques in inbred mice, male mice that lacked a functional aromatase enzyme displayed a marked reduction in aggression. Long-term treatment with estradiol partially restored aggressive behavior, suggesting that the neural conversion of circulating testosterone to estradiol and its effect on [[estrogen receptor]]s influences inter-male aggression. In addition, two different estrogen receptors, [[ERα]] and [[ERβ]], have been identified as having the ability to exert different effects on aggression in mice. However, the effect of estradiol appears to vary depending on the strain of mouse, and in some strains it reduces aggression during long days (16 h of light), while during short days (8 h of light) estradiol rapidly increases aggression.<ref name="Soma2008">{{cite journal |doi=10.1016/j.yfrne.2007.12.003 |pmid=18280561 |title=Novel mechanisms for neuroendocrine regulation of aggression |journal=[[Frontiers in Neuroendocrinology]] |volume=29 |issue=4 |pages=476–89 |year=2008 |last1=Soma |first1=Kiran K. |last2=Scotti |first2=Melissa-Ann L. |last3=Newman |first3=Amy E.M. |last4=Charlier |first4=Thierry D. |last5=Demas |first5=Gregory E. }}</ref>

Another hypothesis is that testosterone influences brain areas that control behavioral reactions. Studies in animal models indicate that aggression is affected by several interconnected cortical and subcortical structures within the so-called [[social behavior]] network. A study involving lesions and electrical-chemical stimulation in rodents and cats revealed that such a neural network consists of the medial [[amygdala]], medial [[hypothalamus]] and [[periaqueductal grey]] (PAG), and it positively modulates reactive aggression.<ref>{{cite journal |doi=10.2174/157015907780866929 |pmid=18615178 |pmc=2435345 |title=The Neurobiological Bases for Development of Pharmacological Treatments of Aggressive Disorders |journal=[[Current Neuropharmacology]] |volume=5 |issue=2 |pages=135–47 |year=2007 |last1=Siegel |first1=Allan |last2=Bhatt |first2=Suresh |last3=Bhatt |first3=Rekha |last4=Zalcman |first4=Steven }}</ref> Moreover, a study done in human subjects showed that prefrontal-amygdala connectivity is modulated by endogenous testosterone during social emotional behavior.<ref>{{cite journal |doi=10.1093/cercor/bhr001 |pmid=21339377 |pmc=3169658 |title=Endogenous Testosterone Modulates Prefrontal-Amygdala Connectivity during Social Emotional Behavior |journal=[[Cerebral Cortex (journal)|Cerebral Cortex]] |volume=21 |issue=10 |pages=2282–90 |year=2011 |last1=Volman |first1=I. |last2=Toni |first2=I. |last3=Verhagen |first3=L. |last4=Roelofs |first4=K. }}</ref>

In human studies, testosterone-aggression research has also focused on the role of the [[orbitofrontal cortex]] (OFC). This brain area is strongly associated with impulse control and self-regulation systems that integrate emotion, motivation, and cognition to guide context-appropriate behavior.<ref name="Mehta, P. H., Beer, J. 2009 2357–2368">{{cite journal |doi=10.1162/jocn.2009.21389 |pmid=19925198 |title=Neural Mechanisms of the Testosterone–Aggression Relation: The Role of Orbitofrontal Cortex |journal=[[Journal of Cognitive Neuroscience]] |volume=22 |issue=10 |pages=2357–68 |year=2010 |last1=Mehta |first1=Pranjal H. |last2=Beer |first2=Jennifer }}</ref> Patients with localized lesions to the OFC engage in heightened reactive aggression.<ref>{{cite journal |doi=10.1176/appi.ajp.2008.07111774 |pmid=18346997 |pmc=4176893 |title=Neurobiology of Aggression and Violence |journal=[[American Journal of Psychiatry]] |volume=165 |issue=4 |pages=429–42 |year=2008 |last1=Siever |first1=Larry J. }}</ref> Aggressive behavior may be regulated by testosterone via reduced medial OFC engagement following social provocation.<ref name="Mehta, P. H., Beer, J. 2009 2357–2368"/> When measuring participants' salivary testosterone, higher levels can predict subsequent aggressive behavioral reactions to unfairness faced during a task. Moreover, brain scanning with [[fMRI]] shows reduced activity in the medial OFC during such reactions. Such findings may suggest that a specific brain region, the OFC, is a key factor in understanding reactive aggression.

====General associations with behavior====
Scientists have for a long time been interested in the relationship between testosterone and aggressive behavior. In most species, males are more aggressive than females. [[Castration]] of males usually has a pacifying effect on aggressive behavior in males. In humans, males engage in crime and especially violent crime more than females. The involvement in crime usually rises in the early teens to mid teens which happen at the same time as testosterone levels rise. Research on the relationship between testosterone and aggression is difficult since the only reliable measurement of brain testosterone is by a [[lumbar puncture]] which is not done for research purposes. Studies therefore have often instead used more unreliable measurements from blood or saliva.<ref name="Handbook of Crime Correlates">{{cite book |doi=10.1016/C2015-0-01432-0 |title=Handbook of Crime Correlates |date=2019 |isbn=978-0-12-804417-9 |first1=Lee |last1=Ellis |first2=David P |last2=Farrington |first3=Anthony W |last3=Hoskin }}{{pn|date=July 2025}}</ref>

''The Handbook of Crime Correlates'', a review of crime studies, states most studies support a link between adult criminality and testosterone although the relationship is modest if examined separately for each sex. However, nearly all studies of juvenile delinquency and testosterone are not significant. Most studies have also found testosterone to be associated with behaviors or personality traits linked with criminality such as [[Antisocial personality disorder|antisocial behavior]] and [[alcoholism]]. Many studies have also been done on the relationship between more general aggressive behavior/feelings and testosterone. About half the studies have found a relationship and about half no relationship.<ref name="Handbook of Crime Correlates"/>

Studies of testosterone levels in male athletes before and after a competition revealed that testosterone levels rise shortly before their matches, as if in anticipation of the competition, and are dependent on the outcome of the event: testosterone levels of winners are high relative to those of losers. No specific response of testosterone levels to competition was observed in female athletes, although a mood difference was noted.<ref name="PMID 10097017">{{cite journal |doi=10.1017/s0140525x98001228 |title=Testosterone and dominance in men |journal=[[Behavioral and Brain Sciences]] |volume=21 |issue=3 |year=1998 |last1=Mazur |first1=Allan |last2=Booth |first2=Alan |pmid=10097017 |pages=353–63; discussion 363–97 }}</ref> In addition, some experiments have failed to find a relationship between testosterone levels and aggression in humans.<ref>{{cite journal |doi=10.1016/S0149-7634(05)80117-4 |pmid=8309650 |title=Aggression in humans: What is its biological foundation? |journal=Neuroscience & Biobehavioral Reviews |volume=17 |issue=4 |pages=405–25 |year=1993 |last1=Albert |first1=D.J. |last2=Walsh |first2=M.L. |last3=Jonik |first3=R.H. }}</ref><ref name=":5">{{cite journal |doi=10.1016/j.jpsychires.2006.04.009 |pmid=16765987 |title=CSF testosterone: Relationship to aggression, impulsivity, and venturesomeness in adult males with personality disorder |journal=[[Journal of Psychiatric Research]] |volume=41 |issue=6 |pages=488–92 |year=2007 |last1=Coccaro |first1=Emil F. |last2=Beresford |first2=Brendan |last3=Minar |first3=Philip |last4=Kaskow |first4=Jon |last5=Geracioti |first5=Thomas }}</ref><ref>{{cite journal |doi=10.1097/00004583-199311000-00015 |pmid=8282667 |title=Testosterone and Aggression in Children |journal=Journal of the American Academy of Child & Adolescent Psychiatry |volume=32 |issue=6  |pages=1217–22 |year=1993 |last1=Constantino |first1=John N. |last2=Grosz |first2=Daniel |last3=Saenger |first3=Paul |last4=Chandler |first4=Donald W. |last5=Nandi |first5=Reena |last6=Earls |first6=Felton J. }}</ref>

The possible correlation between testosterone and aggression could explain the "roid rage" that can result from [[anabolic steroid]] use,<ref>{{cite journal |doi=10.1097/01.wnr.0000234752.03808.b2 |pmid=16957604 |title=Neurosteroids regulate mouse aggression induced by anabolic androgenic steroids |journal=[[NeuroReport]] |volume=17 |issue=14 |pages=1537–41 |year=2006 |last1=Pibiri |first1=Fabio |last2=Nelson |first2=Marianela |last3=Carboni |first3=Giovanni |last4=Pinna |first4=Graziano }}</ref><ref>{{cite journal |doi=10.1002/hup.470050407 |title=High-dose anabolic steroids in strength athletes: Effects upon hostility and aggression |journal=Human Psychopharmacology: Clinical and Experimental |volume=5 |issue=4 |pages=349–56 |year=1990 |last1=Choi |first1=P. Y. L. |last2=Parrott |first2=A. C. |last3=Cowan |first3=D. }}</ref> although an effect of abnormally high levels of steroids does not prove an effect at physiological levels.

===Dehydroepiandrosterone===
[[Dehydroepiandrosterone]] (DHEA) is the most abundant circulating androgen hormone and can be rapidly metabolized within target tissues into potent androgens and estrogens. Gonadal steroids generally regulate aggression during the breeding season, but non-gonadal steroids may regulate aggression during the non-breeding season. Castration of various species in the non-breeding season has no effect on territorial aggression. In several avian studies, circulating DHEA has been found to be elevated in birds during the non-breeding season. These data support the idea that non-breeding birds combine adrenal and/or gonadal DHEA synthesis with neural DHEA metabolism to maintain territorial behavior when gonadal testosterone secretion is low. Similar results have been found in studies involving different strains of rats, mice, and hamsters. DHEA levels also have been studied in humans and may play a role in human aggression. Circulating DHEAS (its sulfated ester) levels rise during adrenarche (≈7 years of age) while plasma testosterone levels are relatively low. This implies that aggression in pre-pubertal children with aggressive conduct disorder might be correlated with plasma DHEAS rather than plasma testosterone, suggesting an important link between DHEAS and human aggressive behavior.<ref name="Soma2008"/>

===Glucocorticoids===
[[Glucocorticoid]] hormones have an important role in regulating aggressive behavior. In adult rats, acute injections of [[corticosterone]] promote aggressive behavior, and acute reduction of corticosterone decreases aggression; however, chronic reduction of corticosterone levels can produce abnormally aggressive behavior. In addition, glucocorticoids affect the development of aggression and the establishment of social hierarchies. Adult mice with low baseline levels of corticosterone are more likely to become dominant than mice with high baseline corticosterone levels.<ref name="Soma2008"/>

Glucocorticoids are released by the [[Hypothalamic–pituitary–adrenal axis|hypothalamic pituitary adrenal]] (HPA) axis in response to [[stress (biological)|stress]], of which [[cortisol]] is the most prominent in humans. Results in adults suggest that reduced levels of cortisol, linked to lower fear or a reduced stress response, can be associated with more aggression. However, it may be that proactive aggression is associated with low cortisol levels while reactive aggression may be accompanied by elevated levels. Differences in assessments of cortisol may also explain a diversity of results, particularly in children.<ref name="van goozen"/>

The HPA axis is related to the general [[fight-or-flight response]] or [[acute stress reaction]], and the role of [[catecholamines]] such as [[epinephrine]], popularly known as adrenaline.

===Pheromones===
In many animals, aggression can be linked to [[pheromone]]s released between [[conspecifics]]. In mice, [[major urinary proteins]] (Mups) have been demonstrated to promote innate aggressive behavior in males,<ref name="BBCreport">{{cite news |title=Aggression protein found in mice |work=[[BBC News]] |date=5 December 2007 |url=https://news.bbc.co.uk/1/hi/sci/tech/7129176.stm |access-date=26 September 2009}}</ref><ref name=Chamero1>{{cite journal |author=Chamero P |title=Identification of protein pheromones that promote aggressive behaviour |journal=Nature |volume=450 |issue=7171 |pages=899–902 |date=December 2007 |pmid=18064011 |doi=10.1038/nature05997 |author2=Marton TF |author3=Logan DW |display-authors=3 |last4=Flanagan |first4=Kelly |last5=Cruz |first5=Jason R. |last6=Saghatelian |first6=Alan |last7=Cravatt |first7=Benjamin F. |last8=Stowers |first8=Lisa|bibcode=2007Natur.450..899C }}</ref> and can be mediated by neuromodulatory systems.<ref>{{cite journal |doi=10.1523/JNEUROSCI.0099-15.2015 |pmid=26224860 |pmc=4518052 |title=Differential Muscarinic Modulation in the Olfactory Bulb |journal=Journal of Neuroscience |volume=35 |issue=30 |pages=10773–85 |year=2015 |last1=Smith |first1=R. S. |last2=Hu |first2=R. |last3=Desouza |first3=A. |last4=Eberly |first4=C. L. |last5=Krahe |first5=K. |last6=Chan |first6=W. |last7=Araneda |first7=R. C. }}</ref> Mups activate [[olfactory sensory neuron]]s in the [[vomeronasal organ]] (VNO), a subsystem of the nose known to detect pheromones via specific [[sensory receptor]]s, of mice<ref name=Chamero1/> and rats.<ref name=Krieger1>{{cite journal |doi=10.1074/jbc.274.8.4655 |pmid=9988702 |title=Selective Activation of G Protein Subtypes in the Vomeronasal Organ upon Stimulation with Urine-derived Compounds |journal=Journal of Biological Chemistry |volume=274 |issue=8 |pages=4655–62 |year=1999 |last1=Krieger |first1=J. |last2=Schmitt |first2=A. |last3=Lobel |first3=D. |last4=Gudermann |first4=T. |last5=Schultz |first5=G. |last6=Breer |first6=H. |last7=Boekhoff |first7=I. |doi-access=free }}</ref> Pheremones have also been identified in [[Drosophila melanogaster|fruit flies]], detected by neurons in the antenna, that send a message to the brain eliciting aggression; it has been noted that aggression pheremones have not been identified in humans.<ref>[https://web.archive.org/web/20091209075216/http://media.caltech.edu/press_releases/13308 Caltech Scientists Discover Aggression-Promoting Pheromone in Flies] Caltech press release, 2009</ref>

==Genetics==
{{Main|Genetics of aggression}}
In general, differences in a continuous phenotype such as aggression are likely to result from the action of a large number of genes, each of small effect, which interact with each other and the environment through development and life.

In a non-mammalian example of genes related to aggression, the [[fruitless]] gene in [[Drosophila melanogaster|fruit flies]] is a critical determinant of certain sexually dimorphic behaviors, and its artificial alteration can result in a reversal of stereotypically male and female patterns of aggression in fighting. However, in what was thought to be a relatively clear case, inherent complexities have been reported in deciphering the connections between interacting genes in an environmental context and a social [[phenotype]] involving multiple behavioral and sensory interactions with another organism.<ref>{{cite journal |doi=10.1016/j.conb.2009.04.001 |pmid=19541474 |pmc=2716404 |title=Fruitless, doublesex and the genetics of social behavior in Drosophila melanogaster |journal=Current Opinion in Neurobiology |volume=19 |issue=2 |pages=200–6 |year=2009 |last1=Siwicki |first1=Kathleen K |last2=Kravitz |first2=Edward A }}</ref>

In mice, candidate genes for differentiating aggression between the sexes are the Sry (sex determining region Y) gene, located on the Y chromosome and the Sts (steroid sulfatase) gene. The Sts gene encodes the steroid sulfatase enzyme, which is pivotal in the regulation of neurosteroid biosynthesis. It is expressed in both sexes, is correlated with levels of aggression among male mice, and increases dramatically in females after [[parturition]] and during [[lactation]], corresponding to the onset of maternal aggression.<ref name="Potegal, M 1996. pp. 869-880"/> At least one study has found a possible epigenetic signature (i.e., decreased methylation at a specific CpG site on the promoter region) of the serotonin receptor 5-HT3a that is associated with maternal aggression among human subjects.<ref name=pmid27720744/>

Mice with experimentally elevated sensitivity to [[oxidative stress]] (through inhibition of copper-zinc superoxide dismutase, [[SOD1]] activity) were tested for aggressive behavior.<ref name="pmid25524980">{{cite journal |vauthors=Garratt M, Brooks RC |title=A genetic reduction in antioxidant function causes elevated aggression in mice |journal=J. Exp. Biol. |volume=218 |issue=Pt 2 |pages=223–7 |date=January 2015 |article-number=jeb.112011 |pmid=25524980 |doi=10.1242/jeb.112011 |doi-access=free }}</ref> Males completely deficient in [[SOD1]] were found to be more aggressive than both wild-type males and males that express 50% of this [[antioxidant]] enzyme. They were also faster to attack another male. The causal connection between SOD1 deficiency and increased aggression is not yet understood.

In humans, there is good evidence that the basic human neural architecture underpinning the potential for flexible aggressive responses is influenced by genes as well as environment. In terms of variation between individual people, more than 100 [[twin studies|twin and adoption studies]] have been conducted in recent decades examining the genetic basis of aggressive behavior and related constructs such as conduct disorders. According to a [[meta-analysis]] published in 2002, approximately 40% of variation between individuals is explained by differences in genes, and 60% by differences in environment (mainly non-shared environmental influences rather than those that would be shared by being raised together). However, such studies have depended on self-report or observation by others including parents, which complicates interpretation of the results.

The few laboratory-based analyses have not found significant amounts of individual variation in aggression explicable by genetic variation in the human population. Furthermore, [[genetic linkage|linkage]] and [[genetic association|association]] studies that seek to identify specific genes, for example that influence neurotransmitter or hormone levels, have generally resulted in contradictory findings characterized by failed attempts at replication. One possible factor is an allele (variant) of the [[Monoamine oxidase A|MAO-A gene]] which, in interaction with certain life events such as childhood maltreatment (which may show a [[main effect]] on its own), can influence development of brain regions such as the amygdala and as a result some types of behavioral response may be more likely. The generally unclear picture has been compared to equally difficult findings obtained in regard to other complex behavioral phenotypes.<ref>{{cite book |last1=Perusse |first1=D |last2=Gendreau |first2=P |chapter=Genetics and the Development of Aggression |editor1-last=Tremblay |editor1-first=Richard Ernest |editor2-last=Hartup |editor2-first=Willard W. |editor3-last=Archer |editor3-first=John |title=Developmental Origins of Aggression |date=2005 |publisher=Guilford Press |isbn=978-1-59385-110-1 }}</ref><ref>{{cite journal |doi=10.1007/s10519-010-9358-9 |pmid=20364435 |pmc=2912157 |title=Harsh Discipline, Childhood Sexual Assault, and MAOA Genotype: An Investigation of Main and Interactive Effects on Diverse Clinical Externalizing Outcomes |journal=Behavior Genetics |volume=40 |issue=5 |pages=639–48 |year=2010 |last1=Derringer |first1=Jaime |last2=Krueger |first2=Robert F. |last3=Irons |first3=Daniel E. |last4=Iacono |first4=William G. }}</ref> For example, both 7R and 5R, [[Attention deficit hyperactivity disorder|ADHD]]-linked VNTR alleles of [[Dopamine receptor D4|dopamine receptor D4 gene]] are directly associated with the incidence of proactive aggression in the men with no history of ADHD.<ref>{{cite journal |doi=10.1016/j.jcrimjus.2015.10.002 |title=Genotype and haplotype frequencies of the DRD4 VNTR polymorphism in the men with no history of ADHD, convicted of violent crimes |journal=Journal of Criminal Justice |volume=43 |issue=6 |pages=464–469 |year=2015 |last1=Cherepkova |first1=Elena V |last2=Maksimov |first2=Vladimir N |last3=Aftanas |first3=Lyubomir I |last4=Menshanov |first4=Petr N}}</ref>

==Society and culture==
[[Humans]] share aspects of aggression with non-human animals, and have specific aspects and complexity related to factors such as genetics, early development, social learning and flexibility, culture and morals.
Konrad Lorenz stated in his 1963 classic, ''[[On Aggression]],'' that human behavior is shaped by four main, survival-seeking animal drives. Taken together, these drives—hunger, fear, reproduction, and aggression—achieve natural selection.{{fact|date=July 2025}} [[E. O. Wilson]] elaborated in ''[[On Human Nature]]'' that aggression is, typically, a means of gaining control over resources. Aggression is, thus, aggravated during times when high population densities generate resource shortages.<ref>E.O. Wilson, ''[[On Human Nature]]'' (Harvard, 1978) pp.101–107.</ref> According to Richard Leakey and his colleagues, aggression in humans has also increased by becoming more interested in ownership and by defending his or her property.<ref>Leakey, R.,& Lewin, R. (1978). ''People of the lake''. New York: Anchor Press/Doubleday.{{pn|date=July 2025}}</ref> However, [[UNESCO]] adopted the Seville [[Seville Statement on Violence|Statement of Violence]] in 1989 that refuted claims, by evolutionary scientists, that genetics by itself was the sole cause of aggression.<ref>{{cite book |last1=Adams |first1=David |title=The Seville Statement on Violence: preparing the ground for the construction of peace |date=1991 |isbn=923-102775-1 |url=https://unesdoc.unesco.org/ark:/48223/pf0000094314 }}{{pn|date=July 2025}}</ref>

Social and cultural aspects may significantly interfere with the distinct expression of aggressiveness. For example, a high population density, when associated with a decrease of available resources, might be a significant intervening variable for the occurrence of violent acts.<ref name="Bond04" />

===Culture===
Culture is one of the factors that influence aggression. [[Tribal]] or [[band society|band]] societies existing before or outside of modern [[state (polity)|states]] have sometimes been depicted as peaceful "[[noble savage]]s". The [[ǃKung people]], for instance, were described as "The Harmless People" in a popular work by [[Elizabeth Marshall Thomas]] in 1958.<ref>{{cite book |last1=Thomas |first1=Elizabeth Marshall |title=The Harmless People |date=1959 |publisher=Vintage Books |isbn=978-0-394-70289-6 }}{{page needed|date=February 2017}}</ref> However, Lawrence Keeley's 1996 [[War Before Civilization]] suggested that regular [[warfare]] without modern technology was conducted by most groups throughout human history, including most [[Indigenous peoples of the Americas|Native American]] tribes.<ref>Keeley, L.H. (1996). ''[[War Before Civilization]]: The myth of the peaceful savage''. New York: Oxford University Press.</ref> 

Research on [[hunter-gatherers]] has revealed a range of behavioral patterns. While aggression, conflict and violence can occur, direct confrontation is generally avoided, and disputes are often managed through various verbal and non-verbal social mechanisms. Different rates of aggression or violence—whether contemporary or historical—have been associated with social structures and environmental factors such as [[resource]] distribution or [[property]] acquisition, [[Land (economics)|land]] use,  [[List of subsistence techniques|subsistence]] strategies, and [[population change|population dynamics]].<ref>{{cite journal |last1=Lomas |first1=William |title=Conflict, Violence, and Conflict Resolution in Hunting and Gathering Societies |journal=Totem: The University of Western Ontario Journal of Anthropology |date=24 June 2011 |volume=17 |issue=1 |url=https://ir.lib.uwo.ca/totem/vol17/iss1/13/ }}</ref>

American psychologist [[Peter Gray (psychologist)|Peter Gray]] hypothesizes that band hunter-gatherer societies are able maintain relatively [[egalitarian]] and peaceful relationships through mechanisms such as fostering a pervasive [[playful]] spirit, the use of humor to deter dominance, and non-coercive, indulgent child-rearing practices. Gray likens hunter-gatherer bands to social play groups, while stressing that such play is not frivolous or even easy at all times.<ref name=PlayFoundation>{{cite journal |author=Gray, Peter |date=Spring 2009 |title=Play as a Foundation for Hunter-Gatherer Social Existence |journal=American Journal of Play |volume=1 |issue=4 |pages=476–522 |url=http://www.journalofplay.org/sites/www.journalofplay.org/files/pdf-articles/1-4-article-hunter-gatherer-social-existence.pdf |archive-url=https://web.archive.org/web/20131024201853/http://www.journalofplay.org/sites/www.journalofplay.org/files/pdf-articles/1-4-article-hunter-gatherer-social-existence.pdf |url-status=dead |archive-date=24 October 2013 }}</ref> According to Gray, "Social play—that is, play involving more than one player—is necessarily egalitarian. It always requires a suspension of aggression and dominance along with a heightened sensitivity to the needs and desires of the other players".<ref>{{cite web |author=Gray, Peter |date=16 May 2011 |title=How Hunter-Gatherers Maintained Their Egalitarian Ways |website=Psychology Today |url=https://www.psychologytoday.com/blog/freedom-learn/201105/how-hunter-gatherers-maintained-their-egalitarian-ways}}</ref>

Joan Durrant of the [[University of Manitoba]] reports that multiple studies have found an association between [[physical punishment]] and higher levels of aggression directed toward parents, siblings, peers, and spouses, even when controlling for other influencing factors.<ref name=Lessons>{{cite journal |doi=10.1503/cmaj.101314 |pmid=22311946 |pmc=3447048 |title=Physical punishment of children: Lessons from 20 years of research |journal=Canadian Medical Association Journal |volume=184 |issue=12 |pages=1373–7 |year=2012 |last1=Durrant |first1=J. |last2=Ensom |first2=R. }}</ref>

Similarly, [[Elizabeth Gershoff]] of the [[University of Texas at Austin]] has found that the more frequently children experience physical punishment, the more likely they are, as adults, to engage in violent behavior toward family members, including intimate partners.<ref>{{cite report |last1=Gershoff |first1=Elizabeth T |title=Report on Physical Punishment in the United States: What Research Tells Us About its Effects on Children |date=2008 |publisher=Center for Effective Discipline |hdl=11212/3568 |hdl-access=free |url=https://www.nospank.net/gershoff.pdf |page=16 }}</ref>

In countries where the physical punishment of children is more culturally accepted, its association with increased aggression tends to be weaker. However, research indicates that physical punishment predicts some increase in child aggression regardless of cultural context.<ref name=SocialSciences>[http://www.encyclopedia.com/doc/1G2-3045300461.html "Corporal Punishment"] (2008). ''International Encyclopedia of the Social Sciences''.</ref>

Although these associations do not establish [[causality]], a number of [[longitudinal study|longitudinal studies]] suggest that experiencing physical punishment can have a direct causal impact on later aggressive behavior.<ref name=Lessons/> In a review of several such studies examining the relationship between disciplinary [[spanking]] and aggression in children from preschool through adolescence, [[Elizabeth Gershoff]] concluded: "Spanking consistently predicted increases in children's aggression over time, regardless of how aggressive children were when the spanking occurred".<ref name=Development>{{cite journal |doi=10.1111/cdep.12038 |pmid=24039629 |pmc=3768154 |title=Spanking and Child Development: We Know Enough Now to Stop Hitting Our Children |journal=Child Development Perspectives |volume=7 |issue=3 |pages=133–137 |year=2013 |last1=Gershoff |first1=Elizabeth T. }}</ref>

Similar findings were reported in a 2010 study led by Catherine Taylor at [[Tulane University]], which identified a link between maternal spanking of three-year-old children and an increased risk of later aggressive behavior.<ref>{{cite journal |doi=10.1542/peds.2009-2678 |pmid=20385647 |pmc=5094178 |title=Mothers' Spanking of 3-Year-Old Children and Subsequent Risk of Children's Aggressive Behavior |journal=Pediatrics |volume=125 |issue=5 |pages=e1057–65 |year=2010 |last1=Taylor |first1=C. A. |last2=Manganello |first2=J. A. |last3=Lee |first3=S. J. |last4=Rice |first4=J. C. }}</ref>

Family violence researcher [[Murray A. Straus]] contends that such evidence has often been overlooked, largely due to a prevailing belief that spanking is more effective than nonviolent forms of discipline. He warns, however, that this belief persists despite evidence of harmful side effects.<ref>{{cite book |author1=Straus, Murray A. |author2=Douglas, Emily M. |author3=Madeiros, Rose Ann |title=The Primordial Violence: Spanking Children, Psychological Development, Violence, and Crime |location=New York |publisher=Routledge |date=2013 |page=81 |isbn=978-1-84872-953-7 |url=https://books.google.com/books?id=gZTCAAAAQBAJ}}</ref>

Cultural and political interpretations of aggression are further complicated by the subjective use of the term "aggressive", which may reflect value judgments depending on the observer's cultural or ideological standpoint.{{according to whom |date=December 2015}} Whether a coercive or violent act is viewed as aggression—or as legitimate or illegitimate aggression—often depends on the social positions of those involved relative to their culture's structure. Relevant factors may include: norms governing coordination and resource distribution; definitions of self-defense and provocation; attitudes toward outsiders and marginalized groups such as women, disabled individuals, or people of lower status; the presence of alternative conflict resolution mechanisms; levels of trade interdependence and participation in [[collective security]] pacts; and broader material or social objectives.<ref name="Bond04">Bond, MH. (2004) 'Aggression and culture', in [https://books.google.com/books?id=UriYBuiH_FkC Encyclopedia of applied psychology], Volume 1.</ref><ref name="Becher Mehlkop Aggressive behavior and social Status">{{cite journal |last1=Becher |first1=Lea |last2=Mehlkop |first2=Guido |last3=Sattler |first3=Sebastian |title=Aggressive behavior and social Status: An experimental test of the general aggression model |journal=Social Science Research |date=September 2025 |volume=131 |article-number=103224 |doi=10.1016/j.ssresearch.2025.103224 }}</ref>

[[Cross-cultural]] research has found differences in attitudes towards aggression in different cultures. In one questionnaire study of university students, in addition to men overall justifying some types of aggression more than women, United States respondents justified defensive physical aggression more readily than Japanese or Spanish respondents, whereas Japanese students preferred direct verbal aggression (but not indirect) more than their American and Spanish counterparts.<ref>{{cite journal |doi=10.1002/(SICI)1098-2337(1999)25:3<185::AID-AB3>3.0.CO;2-K |title=Justification of interpersonal aggression in Japanese, American, and Spanish students |journal=Aggressive Behavior |volume=25 |issue=3 |pages=185–95 |year=1999 |last1=Fujihara |first1=Takehiro |last2=Kohyama |first2=Takaya |last3=Andreu |first3=J. Manuel |last4=Ramirez |first4=J. Martin }}</ref>

Within American culture, men from the [[southern United States]] were found in a study of university students to react more strongly and respond more aggressively than their northern counterparts when randomly insulted after being bumped into. This behavior was theorized to reflect a traditional [[Culture of honor (Southern United States)|culture of honor]] prevalent in the Southern United States, associated with concepts such as "[[Face (sociological concept)|saving face]]."<ref>{{cite journal |doi=10.1037/0022-3514.70.5.945 |pmid=8656339 |title=Insult, aggression, and the southern culture of honor: An "experimental ethnography." |journal=Journal of Personality and Social Psychology |volume=70 |issue=5 |pages=945–59 |year=1996 |last1=Cohen |first1=Dov |last2=Nisbett |first2=Richard E. |last3=Bowdle |first3=Brian F. |last4=Schwarz |first4=Norbert |hdl=2027.42/92155 |hdl-access=free }}</ref>

Other cultural frameworks frequently considered in aggression research include contrasts between [[individualistic]] and [[Collectivism and individualism|collectivist]] orientations. These may influence how individuals respond to disputes—for example, through open competition or by accommodating others and [[Conflict avoidance|avoiding conflict]]. 

In a study including 62 countries, school principals reported higher levels of aggressive student behavior in cultures characterized as more individualistic and, correspondingly, less collectivist.<ref>{{cite journal |doi=10.1002/ab.21472 |pmid=23494751 |title=The Relationship Between Cultural Individualism-Collectivism and Student Aggression Across 62 Countries |journal=Aggressive Behavior |volume=39 |issue=3 |pages=182–200 |year=2013 |last1=Bergmüller |first1=Silvia }}</ref>

Other cross-cultural comparisons related to aggression and conflict have examined differences between [[democracy|democratic]] and [[authoritarianism|authoritarian]] [[political systems]], as well as between [[egalitarian]] and [[social stratification|stratified]] societies.<ref name="Bond04" />

The [[economic system]] known as [[capitalism]] has been interpreted by some scholars as relying on human competitiveness and aggression to drive the pursuit of resources and trade. This perspective has been viewed both positively, as a motivator for innovation and efficiency, and negatively, as a source of inequality and conflict.<ref>Nolan, P. (2007). [https://books.google.com/books/about/Capitalism_and_freedom.html?id=pg6h0_7xRnkC ''Capitalism and Freedom: The Contradictory Character of Globalisation''], p. 2. Anthem Studies in Development and Globalization, Anthem Press.</ref>

Finally, the perceived legitimacy of aggressive acts are influenced by cultural and political attitudes regarding the social acceptability of certain actions or targets. These perceptions are often contested, particularly in cases involving religion or international disputes. One prominent example is the [[Arab–Israeli conflict]], where acts labeled as aggression by one group may be framed as defense or resistance by another.<ref>{{cite journal |doi=10.1016/j.ijintrel.2004.03.004 |title=Aggression and violence among Jewish and Arab Youth in Israel |journal=International Journal of Intercultural Relations |volume=28 |issue=2 |pages=93–109 |year=2004 |last1=Sherer |first1=Moshe |last2=Karnieli-Miller |first2=Orit }}</ref><ref name=amjad>{{cite journal |doi=10.1002/ab.20325 |pmid=19790255 |title=Identifying and changing the normative beliefs about aggression which lead young Muslim adults to join extremist anti-Semitic groups in Pakistan |journal=Aggressive Behavior |volume=35 |issue=6 |pages=514–519 |year=2009 |last1=Amjad |first1=Naumana |last2=Wood |first2=Alex M. }}</ref>

====Media====
{{Main|Media violence research}}
Some researchers argue that behaviors such as aggression may be partially learned through observation and imitation of others, a concept rooted in social learning theory.<ref>Akert, M. Robin, Aronson, E., and Wilson, D.T. "Social Psychology", 5th Edition. Pearson Education, Inc. 2005{{page needed|date=February 2017}}</ref> This perspective includes the view that exposure to media violence may exert small effects on aggressive behavior. However, this remains a subject of ongoing debate, and other scholars have challenged the strength or consistency of such effects.<ref>{{cite book |last1=Freedman |first1=Jonathan L. |title=Media Violence and its Effect on Aggression: Assessing the Scientific Evidence |date=2002 |publisher=University of Toronto Press |isbn=978-0-8020-8425-5 }}{{pn|date=July 2025}}</ref>

For example, a prospective study of adolescents found no long-term relationship between violent video game use and subsequent youth violence or bullying.<ref>{{cite journal |last1=Ferguson |first1=Christopher J. |title=Video Games and Youth Violence: A Prospective Analysis in Adolescents |journal=Journal of Youth and Adolescence |date=April 2011 |volume=40 |issue=4 |pages=377–391 |doi=10.1007/s10964-010-9610-x |pmid=21161351 }}</ref> A meta-analysis concluded that the effect of violent video games on aggression is generally smaller than that found in studies on [[television violence]]. The magnitude of this effect was positively associated with the intensity of violent content and [[Inverse relationship|negatively associated]] with the amount of time spent playing.<ref>{{cite journal |last1=Sherry |first1=Jl |title=The effects of violent video games on aggression..: A meta-analysis |journal=Human Communication Research |date=July 2001 |volume=27 |issue=3 |pages=409–431 |doi=10.1111/j.1468-2958.2001.tb00787.x }}</ref> The study concluded that the available evidence was insufficient to establish a causal relationship between violent video games and increased aggression.

Conversely, other research has reported associations between violent video game exposure and heightened aggressive thoughts, feelings, and behaviors both in laboratory settings and real-life contexts.<ref>{{cite journal |doi=10.1037/0022-3514.78.4.772 |pmid=10794380 |title=Video games and aggressive thoughts, feelings, and behavior in the laboratory and in life |journal=Journal of Personality and Social Psychology |volume=78 |issue=4 |pages=772–90 |year=2000 |last1=Anderson |first1=Craig A. |last2=Dill |first2=Karen E. }}</ref>

===Children===
The frequency of physical aggression in humans peaks at around 2–3 years of age. It then declines gradually on average.<ref>{{cite journal |doi=10.1080/016502500383232 |title=The development of aggressive behaviour during childhood: What have we learned in the past century? |journal=International Journal of Behavioral Development |volume=24 |issue=2 |pages=129–41 |year=2000 |last1=Tremblay |first1=Richard E. }}</ref><ref>{{cite journal |doi=10.1111/j.1467-8624.2004.00755.x |pmid=15369529 |title=Developmental Trajectories of Externalizing Behaviors in Childhood and Adolescence |journal=Child Development |volume=75 |issue=5 |pages=1523–37 |year=2004 |last1=Bongers |first1=Ilja L. |last2=Koot |first2=Hans M. |last3=Van Der Ende |first3=Jan |last4=Verhulst |first4=Frank C. }}</ref> These observations suggest that physical aggression is not only a learned behavior but that development provides opportunities for the learning and biological development of self-regulation. However, a small subset of children fail to acquire all the necessary self-regulatory abilities and tend to show atypical levels of physical aggression across development. They may be at risk for later violent behavior or, conversely, a lack of aggression that may be considered necessary within society.

However, some findings suggest that early aggression does not necessarily lead to aggression later on, although the course through early childhood is an important predictor of outcomes in middle childhood. In addition, physical aggression that continues is likely occurring in the context of family adversity, including socioeconomic factors. Moreover, 'opposition' and 'status violations' in childhood appear to be more strongly linked to social problems in adulthood than simply aggressive antisocial behavior.<ref>{{cite journal |doi=10.1111/j.0037-976X.2004.00312.x |pmid=15667346 |journal=Monographs of the Society for Research in Child Development |volume=69 |issue=4 |pages=vii, 1–129 |year=2004 |author1=NICHD Early Child Care Research Network. |title=Trajectories of physical aggression from toddlerhood to middle childhood: Predictors, correlates, and outcomes |doi-broken-date=2 September 2025 }}</ref><ref>{{cite journal |doi=10.1017/S0033291707002309 |pmid=18047767 |title=Predicting young adult social functioning from developmental trajectories of externalizing behaviour |journal=Psychological Medicine |volume=38 |issue=7 |pages=989–99 |year=2007 |last1=Bongers |first1=I. L. |last2=Koot |first2=H. M. |last3=Van Der Ende |first3=J. |last4=Verhulst |first4=F. C. |url=https://research.vu.nl/en/publications/8b2fcacb-8976-4784-a0f9-429b0d2b87c9 }}</ref> Social learning through interactions in early childhood has been seen as a building block for levels of aggression which play a crucial role in the development of peer relationships in middle childhood.<ref>{{cite journal |last1=Schellenberg |first1=Rita |title=Aggressive Personality: When Does it Develop and Why? |journal=Virginia Counselors Journal |date=2000 |volume=26 |pages=67–76 |url=https://digitalcommons.liberty.edu/educ_fac_pubs/156/ }}</ref> Overall, an interplay of biological, social and environmental factors can be considered.<ref>{{cite book |editor=Tremblay, Richard E |editor2=Hartup, Willard W. |editor3=Archer, John |year=2005 |title=Developmental Origins of Aggression |location=New York |publisher=The Guilford Press |isbn=978-1-59385-110-1|url=https://books.google.com/books?id=XmSfJEl2v4sC}}</ref> Some research indicates that changes in the weather can increase the likelihood of children exhibiting deviant behavior.<ref>{{cite thesis |last1=Dabb |first1=Carrie |title=The Relationship Between Weather and Children's Behavior: A Study of Teacher Perceptions |date=1997 |doi=10.26076/4c1c-66d6 |doi-access=free }}</ref>

====Typical expectations====
* Young children preparing to enter kindergarten need to develop the socially important skill of being [[assertive]]. Examples of assertiveness include asking others for information, initiating conversation, or being able to respond to [[peer pressure]].
* In contrast, some young children use aggressive behavior, such as hitting or biting, as a form of communication.
* Aggressive behavior can impede learning as a skill deficit, while assertive behavior can facilitate learning. However, with young children, aggressive behavior is developmentally appropriate and can lead to opportunities of building conflict resolution and communication skills.
* By school age, children should learn more socially appropriate forms of communicating such as expressing themselves through verbal or written language; if they have not, this behavior may signify a disability or developmental delay.

====Aggression triggers====
* [[Phobia|Physical fear of others]]
* [[Dysfunctional family|Family difficulties]]
* [[learning disorder|Learning]], [[neurological disorder|neurological]], or [[conduct disorder|conduct/behavior disorders]]
* [[Psychological trauma]]

The [[Bobo doll experiment]] was conducted by Albert Bandura in 1961. In this work, Bandura found that children exposed to an aggressive adult model acted more aggressively than those who were exposed to a nonaggressive adult model. This experiment suggests that anyone who comes in contact with and interacts with children can affect the way they react and handle situations.<ref>{{cite journal |doi=10.1037/h0045925 |title=Transmission of aggression through imitation of aggressive models |journal=The Journal of Abnormal and Social Psychology |volume=63 |issue=3 |pages=575–82 |year=1961 |last1=Bandura |first1=Albert |last2=Ross |first2=Dorothea |last3=Ross |first3=Sheila A. |pmid=13864605}}</ref>

;Summary points from recommendations by national associations:
* [[American Academy of Pediatrics]] (2011): "The best way to prevent aggressive behavior is to give your child a stable, secure home life with firm, loving discipline and full-time supervision during the toddler and preschool years. Everyone who cares for your child should be a good role model and agree on the rules he's expected to observe as well as the response to use if he disobeys."<ref>American Academy of Pediatrics (2011) [http://www.healthychildren.org/English/ages-stages/toddler/Pages/Aggressive-Behavior.aspx Ages & Stages: Aggressive Behavior] HealthChildren.org, retrieved January 2012</ref>
* [[National Association of School Psychologists]] (2008): "Proactive aggression is typically reasoned, unemotional, and focused on acquiring some goal. For example, a bully wants peer approval and victim submission, and gang members want status and control. In contrast, reactive aggression is frequently highly emotional and is often the result of biased or deficient cognitive processing on the part of the student."<ref>National Association of School Psychologists (2008) [https://web.archive.org/web/20101205043644/http://www.nasponline.org/resources/principals/Angry%20and%20Aggressive%20Students-NASSP%20Jan%2008.pdf Angry and Aggressive Students]</ref>

===Gender===
{{See also|Sex and psychology}}
Gender is a factor that plays a role in both human and animal aggression. Males are historically believed to be generally more physically aggressive than females from an early age,<ref>Coie, J.D. & Dodge, K.A. (1997). Aggression and antisocial behavior. In W. Damon & N. Eisenberg (Eds). ''Handbook of Child Psychology, Vol. 3: Social, emotional and personality development''</ref><ref>{{cite book |last1=Maccoby |first1=Eleanor E. |last2=Jacklin |first2=Carol Nagy |title=The Psychology of Sex Differences: —Vol. II: Annotated Bibliography |date=1978 |publisher=Stanford University Press |isbn=978-0-8047-0975-0 }}{{pn|date=July 2025}}</ref> and men commit the vast majority of murders (Buss 2005). This is one of the most robust and reliable behavioral sex differences, and it has been found across many different age groups and cultures. However, some [[empirical studies]] have found the discrepancy in male and female aggression to be more pronounced in childhood and the gender difference in adults to be modest when studied in an experimental context.<ref name=pmid3797558/> Still, there is evidence that males are quicker to aggression (Frey et al. 2003) and more likely than females to express their aggression physically.<ref>{{cite journal |last1=Björkqvist |first1=Kaj |last2=Österman |first2=Karin |last3=Lagerspetz |first3=Kirsti M. J. |title=Sex differences in covert aggression among adults |journal=Aggressive Behavior |date=1994 |volume=20 |issue=1 |pages=27–33 |doi=10.1002/1098-2337(1994)20:1<27::aid-ab2480200105>3.0.co;2-q }}</ref> When considering indirect forms of non-violent aggression, such as [[relational aggression]] and [[social rejection]], some scientists argue that females can be quite aggressive, although female aggression is rarely expressed physically.<ref>{{cite journal |doi=10.1037/1089-2680.8.4.291 |title=Sex Differences in Aggression in Real-World Settings: A Meta-Analytic Review |journal=Review of General Psychology |volume=8 |issue=4 |pages=291–322 |year=2004 |last1=Archer |first1=John }}</ref><ref>{{cite journal |doi=10.1111/j.1467-8624.2008.01184.x |pmid=18826521 |title=Direct and Indirect Aggression During Childhood and Adolescence: A Meta-Analytic Review of Gender Differences, Intercorrelations, and Relations to Maladjustment |journal=Child Development |volume=79 |issue=5 |pages=1185–229 |year=2008 |last1=Card |first1=Noel A. |last2=Stucky |first2=Brian D. |last3=Sawalani |first3=Gita M. |last4=Little |first4=Todd D. }}</ref><ref>{{cite journal |doi=10.1891/vivi.2003.18.2.197 |pmid=12816404 |title=Gender Differences in Psychological, Physical, and Sexual Aggression Among College Students Using the Revised Conflict Tactics Scales |journal=Violence and Victims |volume=18 |issue=2 |pages=197–217 |year=2003 |last1=Hines |first1=Denise A. |last2=Saudino |first2=Kimberly J. }}</ref> An exception is [[intimate partner violence]] that occurs among couples who are engaged, married, or in some other form of intimate relationship.

Although females are less likely than males to initiate physical violence, they can express aggression by using a variety of non-physical means. Exactly which method women use to express aggression is something that varies from culture to culture. On [[Bellona Island]], a culture based on male dominance and physical [[violence]], women tend to get into conflicts with other women more frequently than with men. When in conflict with males, instead of using physical means, they make up songs mocking the man, which spread across the island and humiliate him. If a woman wanted to kill a man, she would either convince her male relatives to kill him or hire an assassin. Although these two methods involve physical violence, both are forms of indirect aggression, since the aggressor herself avoids getting directly involved or putting herself in immediate physical danger.<ref name="bjorkqvist">{{cite journal |doi=10.1007/BF01420988 |title=Sex differences in physical, verbal, and indirect aggression: A review of recent research |journal=Sex Roles |volume=30 |issue=3–4 |pages=177–88 |year=1994 |last1=Björkqvist |first1=Kaj }}</ref>

See also the sections on [[#Testosterone|testosterone]] and [[#Evolutionary explanations|evolutionary explanations]] for gender differences above.

===Situational factors===
{{See also|Stereotype threat}}

There has been some links between those prone to violence and their alcohol use. Those who are prone to violence and use alcohol are more likely to carry out violent acts.<ref>{{cite journal |doi=10.1002/ab.20231 |pmid=17922526 |title=Predictors of injurious assault committed during or after drinking alcohol: A case–control study of young offenders |journal=Aggressive Behavior |volume=34 |issue=2 |pages=167–74 |year=2008 |last1=Navis |first1=Charlene |last2=Brown |first2=Stephen L. |last3=Heim |first3=Derek }}</ref> Alcohol impairs judgment, making people much less cautious than they usually are (MacDonald et al. 1996). It also disrupts the way information is processed (Bushman 1993, 1997; Bushman & Cooper 1990).

Recent research further demonstrates that in everyday situations, violations of social norms can be perceived as provocations that trigger aggression,<ref>{{cite journal |last1=Bartholow |first1=Bruce D. |last2=Fabiani |first2=Monica |last3=Gratton |first3=Gabriele |last4=Bettencourt |first4=B. Ann |title=A Psychophysiological Examination of Cognitive Processing of and Affective Responses to Social Expectancy Violations |journal=Psychological Science |date=May 2001 |volume=12 |issue=3 |pages=197–204 |doi=10.1111/1467-9280.00336 |pmid=11437301 }}</ref><ref>{{cite journal |last1=Polman |first1=Evan |last2=Pettit |first2=Nathan C. |last3=Wiesenfeld |first3=Batia M. |title=Effects of wrongdoer status on moral licensing |journal=Journal of Experimental Social Psychology |date=July 2013 |volume=49 |issue=4 |pages=614–623 |doi=10.1016/j.jesp.2013.03.012 }}</ref><ref name="Becher Mehlkop Aggressive behavior and social Status"/> which can have negative consequences for individuals and social coexistence. Based on the General Aggression Model (GAM),<ref name="auto"/><ref>Johnie J. Allen, Craig A. Anderson General aggression model P. Rössler, C.A. Hoffner, L. Zoonen (Eds.), The International Encyclopedia of Media Effects, Wiley (2017), pp. 1-15</ref><ref>{{cite journal |last1=Allen |first1=Johnie J |last2=Anderson |first2=Craig A |last3=Bushman |first3=Brad J |title=The General Aggression Model |journal=Current Opinion in Psychology |date=February 2018 |volume=19 |pages=75–80 |doi=10.1016/j.copsyc.2017.03.034 |pmid=29279227 }}</ref> experimental evidence shows that physical provocation affects the internal state (cognition, arousal, and affect) of the provoked person and shapes their responses.<ref name="Becher Mehlkop Aggressive behavior and social Status"/> The social status of the provocateur can play a moderating role in this process: a higher-status provocateur induces a more aggression-prone internal state, but the likelihood of verbal retaliation is less affected by internal state when provocation comes from high status. These dynamics, illustrate how intentional provocation reduces the probability of ignoring the incident and increases the likelihood of verbal or physical aggression, mediated by changes in the internal state. Overall, this mechanism highlights the importance of situational and social factors—such as status—in everyday aggression and its regulation. This also adds to the currently inconclusive evidence about the role of the social status on aggression and highlights to importance of further inquiry.<ref>{{cite journal |last1=Fragale |first1=Alison R. |last2=Rosen |first2=Benson |last3=Xu |first3=Carol |last4=Merideth |first4=Iryna |title=The higher they are, the harder they fall: The effects of wrongdoer status on observer punishment recommendations and intentionality attributions |journal=Organizational Behavior and Human Decision Processes |date=January 2009 |volume=108 |issue=1 |pages=53–65 |doi=10.1016/j.obhdp.2008.05.002 }}</ref><ref>{{cite journal |last1=Karelaia |first1=Natalia |last2=Keck |first2=Steffen |title=When deviant leaders are punished more than non-leaders: The role of deviance severity |journal=Journal of Experimental Social Psychology |date=September 2013 |volume=49 |issue=5 |pages=783–796 |doi=10.1016/j.jesp.2013.04.003 }}</ref>

Pain and discomfort also increase aggression. Even the simple act of placing one's hands in hot water can cause an aggressive response. Hot temperatures have been implicated as a factor in a number of studies. One study completed in the midst of the civil rights movement found that riots were more likely on hotter days than cooler ones (Carlsmith & Anderson 1979). Students were found to be more aggressive and irritable after taking a test in a hot classroom (Anderson et al. 1996, Rule, et al. 1987). Drivers in cars without air conditioning were also found to be more likely to honk their horns (Kenrick & MacFarlane 1986), which is used as a measure of aggression and has shown links to other factors such as generic symbols of aggression or the visibility of other drivers.<ref>{{cite journal |doi=10.1037/h0076960 |pmid=1142063 |title=Naturalistic studies of aggressive behavior: Aggressive stimuli, victim visibility, and horn honking |journal=Journal of Personality and Social Psychology |volume=31 |issue=6 |pages=1098–107 |year=1975 |last1=Turner |first1=Charles W. |last2=Layton |first2=John F. |last3=Simons |first3=Lynn S. }}</ref>

Frustration is another major cause of aggression.<ref name="pmid2667009">{{cite journal |vauthors=Berkowitz L |title=Frustration-aggression hypothesis: examination and reformulation |journal=Psychol Bull |volume=106 |issue=1 |pages=59–73 |date=July 1989 |pmid=2667009 |doi=10.1037/0033-2909.106.1.59 |url=}}</ref> The [[Frustration aggression theory]] states that aggression increases if a person feels that he or she is being blocked from achieving a goal (Aronson et al. 2005). One study found that the closeness to the goal makes a difference. The study examined people waiting in line and concluded that the 2nd person was more aggressive than the 12th one when someone cut in line (Harris 1974). Unexpected frustration may be another factor. In a separate study to demonstrate how unexpected frustration leads to increased aggression, Kulik & Brown (1979) selected a group of students as volunteers to make calls for charity donations. One group was told that the people they would call would be generous and the collection would be very successful. The other group was given no expectations. The group that expected success was more upset when no one was pledging than the group who did not expect success (everyone actually had horrible success). This research suggests that when an expectation does not materialize (successful collections), unexpected frustration arises which increases aggression.

There is some evidence to suggest that the presence of violent objects such as a gun can trigger aggression. In a study done by [[Leonard Berkowitz]] and Anthony Le Page (1967), college students were made angry and then left in the presence of a gun or badminton racket. They were then led to believe they were delivering electric shocks to another student, as in the [[Milgram experiment]]. Those who had been in the presence of the gun administered more shocks. It is possible that a violence-related stimulus increases the likelihood of aggressive cognitions by activating the [[semantic network]].

A new proposal links military experience to anger and aggression, developing aggressive reactions and investigating these effects on those possessing the traits of a serial killer. Castle and Hensley state, "The military provides the social context where servicemen learn aggression, violence, and murder."<ref>{{cite journal |doi=10.1177/0306624X02464007 |pmid=12150084 |title=Serial Killers with Military Experience: Applying Learning Theory to Serial Murder |journal=International Journal of Offender Therapy and Comparative Criminology |volume=46 |issue=4 |pages=453–65 |year=2002 |last1=Castle |first1=T. |last2=Hensley |first2=C }}</ref> [[Post-traumatic stress disorder]] (PTSD) is also a serious issue in the military, also believed to sometimes lead to aggression in soldiers who are suffering from what they witnessed in battle. They come back to the civilian world and may still be haunted by flashbacks and nightmares, causing severe stress. In addition, it has been claimed that in the rare minority who are claimed to be inclined toward serial killing, violent impulses may be reinforced and refined in war, possibly creating more effective murderers.<ref>{{cite journal|last1=blennow|last2=Manhem|date=20 January 2021|title=CSF studies in violent offenders|url=|journal=Journal of Neural Transmission|volume=108|pages=869–878|via=Forsman A.}}</ref>

===As a positive adaptation theory<!--linked from 'Non-aggression principle'-->===
Some recent scholarship has questioned traditional psychological conceptualizations of aggression as universally negative.<ref name="Ferguson & Beaver, 2009"/> Most traditional psychological definitions of aggression focus on the harm to the recipient of the aggression, implying this is the intent of the aggressor; however this may not always be the case.<ref name="Smith, 2007">{{cite book |last=Smith |first=P. |year=2007 |chapter=Why has aggression been thought of as maladaptive? |chapter-url={{Google books|kfAdzNKaK5AC|page=65|plainurl=yes}} |title=Aggression and Adaptation: the Bright Side to Bad Behavior |pages=65–83 |publisher=Routledge |isbn=978-1-135-59375-9 }}</ref> From this alternate view, although the recipient may or may not be harmed, the perceived intent is to increase the status of the aggressor, not necessarily to harm the recipient.<ref name="Hawley & Vaughn, 2003">{{cite journal |doi=10.1353/mpq.2003.0012 |jstor=23096055 |title=Aggression and Adaptive Functioning: The Bright Side to Bad Behavior |journal=Merrill-Palmer Quarterly |volume=49 |issue=3 |pages=239–42 |year=2003 |last1=Hawley |first1=Patricia H. |last2=Vaughn |first2=Brian E. }}</ref> Such scholars contend that traditional definitions of aggression have no validity because of how challenging it is to study directly.<ref>{{cite book |doi=10.1007/978-0-387-33965-8_2 |chapter=Psychological Theories of Aggression: Principles and Application to Practice |title=Violence in Mental Health Settings |date=2006 |last1=Bjørkly |first1=Stâl |pages=27–46 |isbn=978-0-387-33964-1 }}</ref>

From this view, rather than concepts such as assertiveness, aggression, violence and criminal violence existing as distinct constructs, they exist instead along a continuum with moderate levels of aggression being most adaptive.<ref name="Ferguson & Beaver, 2009"/> Such scholars do not consider this a trivial difference, noting that many traditional researchers' aggression measurements may measure outcomes lower down in the continuum, at levels which are adaptive, yet they generalize their findings to non-adaptive levels of aggression, thus losing precision.<ref name="Ferguson, 2010">{{cite journal |last1=Ferguson |first1=Christopher J. |title=Blazing Angels or Resident Evil? Can Violent Video Games be a Force for Good? |journal=Review of General Psychology |date=June 2010 |volume=14 |issue=2 |pages=68–81 |doi=10.1037/a0018941 }}</ref>

==See also==
{{Div col|colwidth=15em}}
* [[Aggressionism]]
* [[Aggressive narcissism]]
* [[Bullying]]
* [[Child abuse]]
* [[Conflict (disambiguation)]]
* [[Cute aggression]]
* [[Displaced aggression]]
* [[Duelling]]
* [[Frustration-Aggression Hypothesis]]
* [[Hero syndrome]]
* ''[[Homo homini lupus]]''
* [[List of investigational aggression drugs]]
* [[Microaggression]]
* [[Non-aggression pact]]
* [[Non-aggression principle]]
* [[Parental abuse by children]]
* [[Passive aggressive behavior]]
* [[Rage (emotion)]]
* [[Relational aggression]]
* [[Resource holding potential]]
* [[Revenge]]
* [[School bullying]]
* [[School violence]]
* [[Social defeat]]
* [[Violence]]
{{Div col end}}

==References==
{{Reflist}}

==Further reading==
* {{cite journal |last1=Fields |first1=R. Douglas |title=THE ROOTS OF HUMAN AGGRESSION: Experiments in humans and animals have started to identify how violent behaviors begin in the brain |journal=Scientific American |date=May 2019 |volume=320 |issue=5 |pages=65–71 |doi=10.1038/scientificamerican0519-64 |pmid=34276077 |pmc=8284101 |jstor=27265183 }}

== External links ==
{{Wiktionary|aggression|aggressive|aggressiveness}}
{{Wikiquote}}
{{Commons category}}
* [https://web.archive.org/web/20120303110227/http://familylives.org.uk/docs/the_aggression_report_2011_family_lives.pdf When Family Life Hurts: Family experience of aggression in children – Parentline plus, 31 October 2010]
* [http://www.journals.elsevier.com/aggression-and-violent-behavior/ Aggression and Violent Behavior, a Review Journal]
* [http://www.israsociety.com/ International Society for Research on Aggression (ISRA)]
* [http://rint.rechten.rug.nl/rth/dennen/problem1.htm Problems in the Concepts and Definitions of Aggression, Violence and some Related Terms] by Johan van der Dennen, originally published in 1980
* [http://eprints.ucm.es/8423/2/Aggression-assymetries.pdf Aggression and brain asymmetry] {{Webarchive|url=https://web.archive.org/web/20170812220031/http://eprints.ucm.es/8423/2/Aggression-assymetries.pdf |date=12 August 2017 }}

{{Authority control}}
{{Evolutionary psychology}}
{{Nonverbal communication}}

[[Category:Aggression| ]]
[[Category:Behavior]]
[[Category:Broad-concept articles]]
[[Category:Human behavior]]
[[Category:Problem behavior]]
[[Category:Dispute resolution]]
[[Category:Social psychology]]
[[Category:Symptoms and signs of mental disorders]]
[[Category:Violence]]