Neurotransmitters are chemicals that enable impulses within the brain to be transmitted from one area of the brain to another. Serotonin is thought to reduce aggression by inhibiting responses to emotional stimuli that might otherwise lead to an aggressive response. Low serotonin in the brain has been associated with an increased susceptibility to impulsive behaviour, aggression and even violent suicide. A meta-analysis found that serotonin depletion leads to impulsive behaviour which can cause aggression. The second neurotransmitter is dopamine but the dopamine-aggression link is not as well established as with serotonin. Increases in dopamine activity via amphetamines have been associated with more aggression, and antipsychotics reducing dopamine activity have been shown to reduce aggression in violent delinquents. Commentary on serotonin includes evidence from non-human studies.
Support for the importance of serotonin in aggressive behaviour was found in a study of vervet monkeys since individuals fed on diets increasing serotonin in the brain showed lower aggression and vice versa suggesting that aggression can be attributed to serotonin levels. Additionally, selectively bred animals for domestication show a corresponding increase over generations in docile temperaments and concentrations of serotonin. Commentary for serotonin also looks at evidence from antidepressants since if low levels of serotonin are associated with more aggression; drugs which raise serotonin levels should therefore lower aggression. This has been shown to be true since drugs which raise serotonin levels tend to reduce irritability and aggression.
Commentary on dopamine includes that although there is inconclusive evidence on the causal role of dopamine in aggression, new research suggests that it might be a consequence instead, for example, a mice study showed a reward pathway in the brain becomes engaged in response to an aggressive event and that dopamine is involved as a positive reinforcer in this pathway. This suggests that individuals will be aggressive since there is a rewarding sensation. Hormonal mechanisms affecting human aggression include testosterone. Testosterone is an androgen thought to influence aggression from young adulthood onwards due to its action on brain areas in controlling aggression. There have been many research studies on the testosterone-aggression link, for example a study which measured salivary testosterone in criminals found that those with high levels had a history of primarily violent crime, whereas those with low levels had committed only non-violent crime.
Similar trends have been found in studies on non-prison populations. Another study found that young males who behave aggressively when drunk had higher testosterone levels when drunk than those who didn’t act aggressively. A further study showed that male mouse castration reduces aggression but if the mouse is then given testosterone aggression will increase. A weakness of this study is however that the findings are correlational and so we cannot determine causality. Another study found that people who play aggressive sports have higher testosterone levels than players of non-violent sports; this study doesn’t however explain individual differences. One study found that after giving men testosterone a frustrating game increased aggressive responses. However the changes were primarily psychological and there were few behavioural effects.
Finally, a large meta-analysis established a mean correlation of 0.14 between testosterone and aggression; however methodological problems meant that a correlation of 0.08 was more appropriate. Explanations of the testosterone-aggression link also includes the challenge hypothesis since it was proposed that in monogamous species testosterone levels should only rise in response to social challenges such as male-on-male aggression. In such situations there will be a testosterone surge increasing aggression provided that the threat is deemed relevant to reproductive competition, for example a dispute over a female. The second hormone which appears to have a mediating effect on other aggression related hormones, such as testosterone, is cortisol possibly because it increases anxiety and the likelihood of social withdrawal. High levels of cortisol inhibit testosterone levels and so inhibit aggression.
Studies have reported low cortisol levels in violent offenders and violent schoolchildren which suggest that whilst high testosterone is the primary biochemical influence on aggression, low cortisol has an important role in increasing the likelihood of aggressive behaviour. The final group of hormones affecting aggression are female hormones. In the United Kingdom there have been several cases where hormonal fluctuations surrounding pre-menstrual tension have been used for claims of temporary insanity. For example, in 1979 a murder charge was reduced to manslaughter when PMT was accepted as a contributory factor behind the killing. A study found a positive correlation between levels of androgens in the body and aggressive behaviour in both female and male prisoners.
A criticism of this study is that androgen levels weren’t measured at the time of aggression and so it isn’t clear is they were the only variable affecting behaviour. Commentary on the testosterone-aggression link includes that there is inconsistent evidence since some studies have found no link, particularly those comparing testosterone levels of aggressive and less-aggressive individuals. Most studies showing a positive correlation have used small samples of men in prison using either self-report measures of aggression or judgements based solely on the crime committed. Further commentary on testosterone includes that the research needs to distinguish between aggression and dominance. Aggression is when there is the intent of inflicting injury whereas dominance if when there is the wish of maintaining status over another.
It has been suggested that aggression is just one form of dominance and that in non-humans the influence of testosterone on dominance behaviour might be shown in aggressive behaviour. But in humans the influence of testosterone on dominance is likely to be expressed in more varied and subtle ways. Commentary on the cortisol-aggression link includes support for its moderating effect since a study of boys with behavioural problems found that low cortisol meant earlier antisocial acts and more aggressive symptoms showing that cortisol levels are strongly and inversely related to aggression. General commentary on the neural and hormonal aggression link includes reductionism.
This is because whilst the link between biological mechanism and aggression is well established in non-humans, humans are more complex and therefore biological factors represent an incomplete picture. There is also gender bias to the research since it tends to focus on males despite the fact that studies of females also show an important role for testosterone.
There are also ethical issues with socially sensitive research. Finally the research has real world applications, for example gun crime has increased perhaps because there are more guns and they are seen as a threat which increases aggression. Also in a study where males gave a saliva sample and then played with a child’s toy or a gun for fifteen minutes and then gave a second sample showed that those who interacted with the gun showed more testosterone and aggression than those who played with the child’s toy.