Neural and hormonal mechanisms in aggression

• Limbic system are the subcortical structures in the brain including the hypothalamus and amygdala thought to be closely involved in regulating emotional behaviour including aggression. The reactivity of the amygdala in humans and other mammals is an important predictor of aggressive behaviour 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.
• This amygdala activity is illustrated in a study by Katarina Gospic et al. (2011) where some participants were subjected to mild provocation. When participants reacted aggressively fMRI scans showed a fast and heightened response by the amygdala. A benzodiazepine drug which reduces arousal of the autonomic nervous system taken before the provocation led to two effects: it decreased the activity of the amygdala and halved the number of rejections meaning reduced aggression, illustrating again the role of the amygdala.
• 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 this mechanism, reducing self-control and leading to an increase in impulsive behaviour including aggression (Denson et al. 2012).
• Matti Virkkunen et al. (1994) compared levels of a serotonin breakdown product in the cerebrospinal fluid of violent impulsive and violent non-impulsive offenders. The levels were significantly lower in the impulsive offenders. The male sex hormone testosterone is an androgen responsible for the development of masculine features. It is thought to be linked to aggressive behaviour for several reasons. Men become more aggressive towards other men at a time in development after age 20 years when testosterone levels are highest this was found by Daly and Wilson 1988.
• Testosterone has a role in regulating social behaviour via its influence on certain areas of the brain implicated in aggression. 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 this was found by Giammanco et al. (2005).
• Some evidence for a similar association in humans comes from studies of prison populations, for example violent offenders. Mairead 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.
• One limitation of the limbic explanation is more recent research showing that non-limbic brain structures are also involved in aggression. Limbic structures like the amygdala function together with the orbitofrontal cortex (OFC), which is not part of the limbic system. The OFC is involved in impulse regulation and inhibition of aggressive behaviour.
• According to Emil Coccaro et al. (2007), OFC activity is reduced in those psychiatric disorders that feature aggression. This reduced activity disrupts the OFC's impulse-control function, which in turn causes aggressive behaviour. This shows that the neural regulation of aggression is more complex than theories focusing on the amygdala suggest.
• One strength of the serotonin explanation is research into the effects of drugs. Drugs like paroxetine that increase serotonin have been found to also reduce levels of aggressive behaviour. Mitchell Berman et al. (2009) gave participants either a placebo or a dose of paroxetine. The participants then took part in a lab-based game that involved giving and receiving electric shocks in response to provocation. The paroxetine group consistently gave fewer and less intense shocks than the placebo group. This study is evidence of a causal link between serotonin function and aggression.
• One limitation is mixed evidence of the link between testosterone and aggression in humans. Justin Carré and Pranjal Mehta (2011) developed a dual-hormone hypothesis to explain why. They claim that high levels of testosterone lead to aggressive behaviour but only when levels of cortisol are low. When cortisol is high, testosterone's influence on aggression is blocked. The hormone cortisol plays a central role in the body's response to chronic stress. Therefore, the combined activity of testosterone and cortisol may be a better predictor of aggression than either hormone alone.