Editing Aggression (section)

===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">Carlson, N. 'Hormonal Control of Aggressive Behavior' Chapter 11 in [Physiology of Behavior],2013, Pearson Education Inc.</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">Van Goozen, S. 'Hormones and the Developmental Origins of Aggression' Chapter 14 in [https://books.google.com/books/about/Developmental_origins_of_aggression.html?id=XmSfJEl2v4sC Developmental Origins of Aggression], 2005, The Guilford Press.</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 |s2cid=8041587 }}</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|s2cid=26405251 }}</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. |s2cid=32650274 }}</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 |citeseerx=10.1.1.518.2751 |s2cid=710598 }}</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=Ellis2009>''Handbook of Crime Correlates''; Lee Ellis, [[Kevin M. Beaver]], [[John Paul Wright|John Wright]]; 2009; Academic Press</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=Ellis2009/>

Studies of testosterone levels of 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|citeseerx=10.1.1.421.3005 |s2cid=9462611 }}</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. |s2cid=28557481 }}</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 |s2cid=42991833 }}</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. |s2cid=37157824 }}</ref> although an effect of abnormally high levels of steroids does not prove an effect at physiological levels.