neural and hormonal mechanisms

• Limbic system

  Subcortical structures in the brain (including the hypothalamus and amygdala) thought to be closely involved in regulating emotional behaviour including aggression.
• Testosterone

  A hormone from the androgen group that is produced mainly in the male testes (and in smaller amounts in the female ovaries)
• Neural mechanisms in aggression: The limbic system

  Papez (1937) and Maclean (1952) linked the limbic system to emotional behaviours, including aggression.
• Papez (1937) and Maclean (1952)

  They defined the limbic system as the hypothalamus, amygdala and parts of the hippocampus. The reactivity of the amygdala in humans and other mammals is an important predictor of aggressive behaviour.
• Papez (1937) and Maclean (1952)


  The more responsive the amygdala, the more aggressive a person is. So the amygdala has a key role in how mammals assess and respond to environmental threats.
• Gospic et al. (2011) did a study on amygdala activity
• In Gospic et al. (2011) study some participants were subjected to mild provocation. When participants reacted aggressively fMRI scans they showed a fast and heightened response by the amygdala.
• Gospic et al. (2011) had some participants take a benzodiazepine drug (which reduces arousal of the autonomic nervous system) before the provocation. This led to two effects:

  1. It decreased the activity of the amygdala
  2. Halved the number of rejections (i.e. reduced aggression)
• Serotonin is a neurotransmitter that has widespread inhibitory effects in the brain, i.e. it slows down and calms neuronal activity.
• Normal levels of serotonin in the orbitofrontal cortex (OFC) are linked with reduced firing of neurons, which in turn is associated with greater behavioural self-control.
• Decreased serotonin disrupts this mechanism, reducing self-control and leading to an increase in impulsive behaviour including aggression (Denson et al. 2012)
• Virkkunen et al. (1994) compared levels of a serotonin breakdown product (a metabolite called 5-HIAA) in the cerebrospinal fluid of violent impulsive and violent non-impulsive offenders. The levels were significantly lower in the impulsive offenders.
• Men become more aggressive towards other men after age 20 years when testosterone levels are highest (Daly and Wilson 1988)
• Castration studies of animals show that removing the testes (the source of testosterone) reduces aggression in the males of many species. Giving injections of testosterone to the same animals restores aggressive behaviour (e.g. Giammanco et al. 2005)
• Studies of prison populations - Dolan et al. (2001) found a positive correlation between testosterone levels and aggressive behaviours in a sample of 60 male offenders in UK maximum security hospitals. These men mostly had personality disorders (such as psychopathy) and histories of impulsively violent behaviour.
• There is some evidence that progesterone (a female ovarian hormone) plays an important role in aggression in women.
• Levels of progesterone vary during the ovulation cycle and are lowest during and just after menstruation.
• Ziomkiewicz et al. (2012) found a negative correlation between progesterone levels and self-reported aggression. This suggests that low levels of progesterone are linked to increased aggression in women.