Neural and Hormonal Mechanisms in Aggression

• Serotonin is thought to reduce aggression by inhibiting responses to emotional stimuli that might otherwise lead to an aggressive response
• Low levels of serotonin in the brain have been linked to an increased susceptibility to impulsive behaviour, aggression, and even violent suicide
• Some drugs are thought to alter serotonin levels and thus increase aggressive behaviour

• Mann et al: gave 35 healthy subjects dexfenfluramine (known to deplete serotonin)
• Used a questionnaire to assess hostility and aggression levels
• Found that dexfenfluramine treatment in males (but not females) was associated with an increase in hostility and aggression scores

• The link between high levels of dopamine and aggressive behaviour is not as well-established as the link with serotonin
• However, there is some evidence to suggest that a link exists
• Lavine: increases in dopamine activity by the use of amphetamines have also been associated with increases in aggressive behaviour
• Buitelaar: antipsychotics, which reduce dopamine activity in the brain, have been shown to reduce aggressive behaviour in violent delinquents

• Scerbo and Raine: meta-analysis of 29 studies carried out pre-1992
• Examined neurotransmitter levels in antisocial children and adults
• Studies consistently found lower levels of serotonin in individuals described as being aggressive, but found no significant rise or fall in dopamine levels
• Indications of reduced levels of serotonin were found in all antisocial groups, but were particularly marked in those who had attempted suicide
• This suggests that serotonin depletion leads to impulsive behaviour, which in turn may lead to aggressive behaviour in various forms

Research studies

• Dabbs et al: measured salivary testosterone in violent and non-violent criminals
• Those with the highest testosterone levels had a history of primarily violent crime, whereas those with the lowest levels had committed only non-violent crimes
• Lindman et al: found that young males who behaved aggressively when drunk had higher testosterone levels than those who did not

The challenge hypothesis

• Wingfield et al: in monogamous species, testosterone levels should only rise above the baseline breeding level in response to social challenges
• These include male-male aggression or threats to status
• As the human species is considered to be monogamous, this would predict that male testosterone levels would rise sharply in response to such challenges
• A testosterone surge is to be expected, with a consequent increase in aggression, provided the threat is deemed relevant to reproductive competition

• Cortisol appears to have a mediating effect on other aggression-related hormones, possibly because it increases anxiety and the likelihood of social withdrawal
• High levels of cortisol inhibit testosterone levels and so inhibit aggression
• Virkkunen: studies have reported low levels of cortisol in habitual violent offenders
• Tennes and Kreye: this has also been noticed in violent schoolchildren
• This suggests that although relatively high testosterone is the primary biochemical influence on aggression, low cortisol increases the likelihood of aggressive behaviour

• Two meta-analyses have established a weak but positive relationship

• Archer: analysed the results of 230 males over 5 studies
• Found a low positive correlation between testosterone and aggression
• The type of participant and the form/measurement of aggression did vary across these five studies, however

• Book et al: analysed 45 studies
• Established a mean correlation of +0.14 between testosterone and aggression
• Archer: claims that methodological problems with this study meant that a correlation of +0.08 was more appropriate

• SUPPORTS
• Raleigh et al: studied vervet monkeys
• Found that individuals fed on experimental diets high in tryptophan (which increases serotonin levels) exhibited decreased levels of aggression
• Individuals fed on diets low in tryptophan exhibited aggressive behaviour, suggesting that the difference in aggression could be attributed to their serotonin levels
• Popova et al: studied animals selectively bred for domestication and for increasingly docile temperaments
• Showed an increase, over generations, in brain concentrations of serotonin

• SUPPORTS
• If low levels of serotonin are associated with low impulse control and aggressive behaviour, drugs that clinically raise serotonin levels should lower aggression
• Bond: this is exactly what happened in clinical studies of antidepressant drugs that elevate serotonin levels
• Such drugs do tend to reduce irritability and impulsive aggression

• UNDERMINES
• Although research is fairly inconclusive about the causal role of dopamine in aggression, recent research suggests that its influence might be as a consequence instead
• Couppis and Kennedy: in mice, a reward pathway in the brain becomes engaged in response to an aggressive event
• Dopamine is a positive reinforcement in this pathway
• Suggests that individuals will intentionally seek out aggressive encounters because they experience a rewarding sensation from it

• UNDERMINES
• Albert et al: claim that many studies show no correlation between testosterone and aggression, especially those that compared testosterone in more/less aggressive people
• The studies that showed a positive correlation used small samples of men within prisons
• Used either self-report measures of aggression or judgments based solely on the severity of the crime committed

• UNDERMINES
• Mazur: we should distinguish aggression from dominance
• Aggression = intent is to inflict injury
• Dominance = intent is to achieve or maintain status over another person
• Aggression could be just one form of dominance behaviour
• In non-human animals, the influence of testosterone on dominance might be shown as aggressive behaviour
• In humans, it is likely to be expressed in more varied and subtle ways, e.g. through status-striving behaviour

• SUPPORTS
• The moderating effect of cortisol on aggressive behaviour is supported in a four-year study of boys with behavioural problems
• McBurnett et al: the boys with consistently low cortisol levels began antisocial acts at a younger age, and exhibited 3x more aggressive symptoms than boys with higher levels
• Researchers concluded that cortisol levels were 'strongly and inversely related to aggressive conduct disorder'

• UNDERMINES
• The links between biological mechanisms such as serotonin and aggression, and testosterone and aggression, are well-established in non-human animals
• However, the position is not quite so clear in the case of humans
• The complexity of human social behaviour means that a biological explanation for human aggression is insignificant on its own to explain the many aspects of aggression

• SUPPORTS
• Statistics suggest a sharp increase in gun-related crime in the UK, but why does the presence of guns in the environment lead to increased aggression?
• Perhaps the presence of a stimulus such as a gun or knife triggers increases in testosterone levels (the gun is seen as a threat), which in turn increases aggression

• Klinesmith et al: had male college students provide a saliva sample (to measure testosterone), interact with a toy or real gun for 15 minutes, then give another sample
• Males who had interacted with the gun showed significantly greater increases in testosterone and behaved more aggressively towards another participant

• SUPPORTS
• The majority of studies look into the link between aggression and testosterone in males
• Archer et al: suggests that this association is higher for female than male samples
• Baucom et al: women with higher testosterone levels had higher occupational status, possibly as a result of being more assertive
• These studies indicate that women may be responding to challenging situations with increased testosterone, displaying characteristics such as aggression and dominance
• In some circumstances this can be a disadvantage: this assertive style hinders the formation of alliances as well as the more subtle forms of competition in female groups