Neural and Hormonal Mechanisms in Aggression

Neural and Hormonal Mechanisms in Aggression, AO1 and AO2.

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AO1 - Neurotransmitters: Serotonin

  • 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
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AO1 - Neurotransmitters: Dopamine

  • 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
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AO1 - Neurotransmitters and Aggression Link

  • 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
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AO1 - Hormones: Testosterone

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
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AO1 - Hormones: Cortisol

  • 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
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AO1 - Testosterone and Aggression Link

  • 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
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AO2 - Serotonin: Evidence from Non-Human Studies

  • 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
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AO2 - Serotonin: Evidence from Antidepressants

  • 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
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AO2 - Dopamine

  • 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
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AO2 - Testosterone: Inconsistent Evidence

  • 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
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AO2 - Testosterone: Aggression or Dominance?

  • 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
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AO2 - Cortisol

  • 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'
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IDA - 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 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
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IDA - Real-World Applications

  • 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
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IDA - Gender Bias

  • 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
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