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Neural and Hormonal Mechanisms in Aggression
Transcript of Neural and Hormonal Mechanisms in Aggression
Increases in dopamine levels have been shown to produce increases in aggressive behaviour.
Demonstrated in studies that used amphetamines (to increase dopamine activity in the brain) and found an associated increase in levels of aggressive behaviour (e.g. Lavine)
Studies that have reduced dopamine levels through the use of antipsychotic drugs have reported a reduction in aggressive behaviour (Buitelaar)
Low levels of serotonin are associated with aggressive behaviour.
Serotonin usually reduces aggression by inhibiting responses to stimuli that might otherwise lead to aggressive behaviour.
Low levels of serotonin have been linked to an increased susceptibility to impulsive and aggressive behaviour.
Drugs that secrete serotonin (e.g. dexenfluramine) have been shown to increase aggression levels in participants (Mann et al.)
A meta-analysis of studies published before 1992 (Scerbo and Raine) found lower levels of serotonin in individuals described as being aggressive. (supports claim that serotonin depletion leads to impulsive behaviour, which may lead to aggression).
Evidence for the importance of serotonin comes form studies of animals that have been specially bred for domestication and for increasingly docile temperaments . These animals have shown a corresponding increase, over generations, of levels of serotonin in the brain. (e.g. Rayleigh et al. 1991, Popova et al. 1991)
If drugs that deplete serotonin activity lead to raised aggression levels, then drugs that raise it should lead to lower levels. This is what was found in clinical studies of antidepressant drugs, further supporting the link between serotonin and aggression.
Evidence for the causal role played by dopamine in aggression is inconclusive, but a study by Couppis and Kennedy suggests that it may be a consequence rather than a cause. Some individuals may seek out aggressive encounters because dopamine is released as a positive reinforcer in the rain when they engage in aggressive behaviours.
A meta-analysis (Scerbo and Raine) examined neurotransmitter levels in antisocial children and adults. They found lower levels of serotonin in individuals described as aggressive, but found no significant rise or fall in dopamine levels for this group compared to 'normal' individuals.
Testosterone is thought to increase aggression in adults due to its action on brain areas involved in controlling aggression.
Studies (e.g. Dabbs) have found that criminals with the highest levels of testosterone tended to have a history of mainly violent crimes, whereas those with the lowest levels had committed mainly non-violent crimes.
The challenge hypothesis proposes that testosterone would only rise above baseline level in response to social challenges (e.g. to status), with a resultant rise in aggressive behaviour.
Although many studies show a link between testosterone and aggressive behavior, many others do not. Most of the studies that do find a positive correlation between testosterone levels and aggression have used small samples of men within prisons relying on self-reports of aggressive behaviour.
IDA - Most studies of the links between testosterone and aggression have involved male participants, yet research suggests that the link between testosterone and aggression may be even stronger for females (Archer et al.) Studies suggest that successful career women have higher testosterone levels.
Mazur claims that aggression is just one form of dominance behaviour. In humans, the influence of testosterone on dominance is likely to be expressed in more varied and subtle ways rather than only through aggressive behaviour.
High levels of cortisol inhibit testosterone levels and so inhibit aggression.
Low levels of cortisol have been reported in habitual violent offenders (Virkkunen) and violent school children (Tennes and Kreye).
This suggests that, although testosterone may be the main biochemical influence on aggressive behaviour, low cortisol levels increase the likelihood of aggressive behaviour.
The moderating effect of cortisol on aggressive behaviour is supported by research with boys with aggressive conduct disorder (McBurnett et al.). Boys with consistently low cortisol levels had three times the level of aggressive symptoms compared to boys with high or fluctuating levels of cortisol. This demonstrates that cortisol levels are strongly and inversely related to levels of aggression.
One possible reason is that boys who have low cortisol levels might be less afraid of punishment. As a result, such children may not experience anxiety at the threat of punishment and so do not avoid aggressive situations in the same way as do other children.