Neural and Hormonal Mechanisms in Aggression

• Low levels of serotonin and high levels of dopamine have been associated with aggression in animals and humans.
• Serotonin- serotonin normally inhibits responses to stimuli that might lead to aggressive behaviour and in some cases violent suicide.
• Mann et al., 1990 gave 35 subjects dexenfluramine(a drug that depletes serotonin). They used a questionnaire to assess hostility and aggression levels. it was found that dexenfluramine treatment was associated with an increase of aggressive behaviour and hostility, but only in men.
• Dopamine- the link between high levels of dopamine and aggression is not well established but there is some evidence to suggest that such a link does exist.
• Lavine 1997 found that increases in dopamine activity via the use of amphetamines have also been associated with aggressive behaviour.
• Buitlaar 2003 found that antipsychotics that reduce dopamine activity in the brain have been shown to reduce aggressive behaviour in violent delinquents.

• To evaluate, evidence to support the role of serotonin in aggression was found by Scerbo and Raine 1993. They conducted a meta-analysis of 29 studies that examined neurotransmitter levels in antisocial children and adults. the studies consistently found lower levels of serotonin in those described as being aggressive. This was the case in all antisocial groups, but were particularly marked in those who had attempted suicide.
• Raleigh et al., 1991 have added support for the link between serotonin and aggressive behaviour in a study of vervet monkeys. they found that the group fed on diets high in tryptophan exhibited decreased levels of aggression. the group fed on diets that were low in tryptophan exhibited increased aggressive behaviour, suggesting the difference in aggression could be attributed to their serotonin levels.
• Bond 2005 has established that antidepressant drugs that elevate serotonin levels tend to reduce irritability and impulsive aggression.
• To challenge the claim that high levels of dopamine are associated with aggression, recent research suggests that the causal role of dopamine in aggression might be consequence instead. Couppis and Kennedy 2008 found that in mice, a reward pathway in the brain becomes engaged in response to an aggressive event and dopamine is the positive reinforcer. this suggests that a person may seek out an aggressive encounter soley because they get a rewarding sensation from it.

• The male sex hormone testosterone is thought to influence aggression due to its action on the brain areas involved in controlling aggression.
• Dabbs et al., 1987 measured salivary testosterone in violent and non-violent criminals. those who had highest levels of testosterone had a history of violent crimes, whereas those with low levels had committed only non-violent crimes. studies of non-prison populations have found similar trends.
• Lindman et al., 1987 found that young males who behaved aggressively when drunk had higher testosterone levels than those who did not act aggressively.
• The challenge hypothesis- Wingfield et al., 1990 proposes that, in monogonous species, testosterone levels should only rise above the baseline breeding level in response to social challenges such as male-male aggression or threats to status. Provided that the threat is relevant to reproductive competition.
• Dabbs et al., 1991 found that high levels of cortisol inhibit testosterone and so inhibit aggression.
• Vukkenen 1985 found that studies have reported low levels of cortisol in habitual violent offenders and in violent school children.

• to evaluate, the evidence supporting the role of testosterone in aggression is inconsistent.
• Albert et al., 1995 claim that despite many studies showing a positive correlation between testosterone and aggression, other studies find no such relationship. also, most studies that did find a positive correlation have involved small samples of men within prisons, using either self- report measures of aggression or judgments based soley on the serenity of the crime committed.
• Mazur 1985 also challenges the link between testosterone and aggression. He suggested that influence of testosterone is limited to dominance rather than aggression. He claims that a person acts aggressively when their intent is to inflict injury, whereas they act dominantly if their wish is to achieve or maintain status over another.
• McBurnett et al., 2000 supports the effect of cortisol on aggressive behaviour in a four year study of boys with behavioural problems. boys with consistently low levels of cortisol began antisocial acts at a younger age and exhibited three times the number of aggressive symptoms compared to boys with higher levels of cortisol. it has been concluded that cortisol levels are strongly related to aggressive conduct disorder.

Reductionism and biological mechanisms- the links between biological mechanisms such as serotonin and aggression, and testosterone and aggression are well established in non-human animals. However, the case is not so clear in humans. the complexity of human social behaviour means that a biological explanation for human aggression is insufficient on its own to explain all the man different aspects of aggressive and violent behaviour.

Gender bias- An issue raised within this area of study is gender bias. Most studies concerned with testosterone and aggression have involved male participants. Archer et al., 2005 suggests that the association between testosterone and aggression is higher for female than male samples. Baucum et al., 1985 found that women with higher testosterone levels had higher occupational status, possibly as a result of being more assertive. This supports the challenge hypothesis as these studies indicate that women may also respond to challenging situations with increased testosterone, displaying aggressive and dominant characteristics.