Neural Mechanisms in aggression

Neural Mechanisms in aggression

1. Neurotransmitters

   • Neurotransmitters are chemicals which trigger a response in the brain. For example serotonin and dopamine are though to influence aggression levels
   • Serotonin
     • Low levels of serotonin (the bodies natural "happy drug" can lead to an increase in aggressive behaviour.
     • Serotonin is thought to inhabit our response to emotional stimuli that might otherwise lead to aggressive behaviour, so low levels of serotonin means we will respond aggressively more often
       • Mann et al (1990) gave 35 healthy subjects dexfenfluramine, which is known to deplete serotonin. Using a questionnaire to assess hostility and aggression levels, they found that dexfenfluramine treatment in males (but not females) was associated with an increase in hostility and aggression score.
     • Evaluation
       • Evidence from non-human studies
         • Raleigh et al(1991) supports the importance of serotonin. A study on vervet monkeys where individuals fed on experiment diets high in trypophan(a drug which increases serotonin) exhibited decreased levels of aggression. Individuals fed low levels of trypophan had increased aggressive behviour, this suggests that the difference in aggression could be linked to serotonin levels.
         • Popova et al(1991) found that in animals selective breeding for domestication and for increase in dolice temperaments have also seen an increase over the generation of the brain concentration of serotonin
       • Evicence from antidepressants
         • If low levels of serotonin are associated with low impulse control and aggressive behaviour, drugs that clinically raise serotonin levels should produce a concurrent lowering in aggression.
           • Bond (2005) has established that this is exactly what happens in clincal studies of antidepressant drugs that elevate serotonin levles. She established that such drugs tend to reduce irriability and impulsive aggression.
   • Dopamine
     • The link between high levels of dopamine and aggressive behaviour is not well established. Research has found a relationship between the two
     • Lavine (1997) found that an increase in dopamine levels though the use of amphetamines was associated with an increase in aggressive behaviour, suggesting that higher levels of dopamine correlate with higher levels of aggression
     • Buitelaar(2003) found that antipsychotics, which reduce dopamine activity in the brain, have been shown to reduce aggressive behaviour in violent delinquents.
     • Evaluation
       • Research is fairly conclusive about the casual role of dopamine in aggression, recent research suggests that its influence might be as a consequence instead.
         • Couppis and Kennedy (2008) found that in mice, a reward pathway in the brain becomes engaged in response to an aggressive event that dopamine is involved as a postive reinforcer in this pathway. This suggests that individuals will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it.

2. Hormonal Mechanisms

   • Testosterone
     • Testosterone is the male sex hormone and is thought to influence aggression as it affects the area of the brain which are involved in controlling aggression.
     • Research studies
       • Dabbs et al(1987) took saliva samples of criminals to test levels of testosterone. They found that those with higher levels of testosterone had a history of violent crime, whereas those with low levels of testosterone had committed only non-violent crimes.
       • Lindman et al(1987) found that young males who behaved aggressively when drunk had higher levels of testosterone than those who did not act aggressively
     • The challenge hypothesis
       • Wingfield et al (1990), in monogamous species , testosterone levels should rise above the baseline breeding levels in response to social challenges , such as threats to status. The human species is considered monogamous, which would predict that the male testosterone levels would rise sharply in response to challenges. In such situations testosterone surges would be expected, with a consequent increase in aggression, provided the treat is deemed relevant to reproductive competition e.g. a dispute over a female
     • Evaluation
       • Inconsistent evidence
         • Albert et al (1993) claim that despite many studies show a positive correlation between testosterone and aggression, many studies don't find a correlation. Particularly those which compare testosterone levels of aggressive and less aggressive individuals. Most studies showing a positive correlation have involve a small sample of men in prisons, using either self-report measures of aggression or judgement based solely on the severity of the crime committed
       • Aggression or dominance?
         • Mazur (1985) suggests we should distinguish aggression from dominance. Individuals act aggressively when their intent is to inflict injury, whereas they act dominantly if their wish is to achieve or maintain status over another. Mazur claims that aggression is just one form of dominance behaviour. In non-humans the influence of testosterone on dominance behaviour might be shown in aggressive behaviour. In humans, the influence of testosterone on dominance is likely to be expressed in more varied and subtle ways.
   • Cortisol
     • Cortisol appears to have a mediating effect on other aggression-related hormones such as testosterone, possibly because it increases anxiety and the likelihood of social withdrawal.
     • Dabbs et al found that high levels of cortisol inhibit testosterone and this in turn inhabits aggression. So by inhibiting testosterone, cortisol can affect aggression levels.
     • Vukkenen (1985) found that studies have reported low levels of cortisol in habitual violent offenders and in violent school children, this supports the theory that high levels of cortisol reduces aggression.
     • Evaluation
       • The moderating effect of cortisol on aggressive behaviour is supported in a four year study of boys with behaviour problems. McBurnett (2000) found that in those boys with consistently low cortisol levels displayed antisocial acts at a younger age. They were 3 times more aggressive symptoms compared to the boys with high cortisol levels. Research concludes that cortisol levels were "strongly and inversely related to aggressive conduct disorder."