Neural and hormonal mechanisms in aggression
The brain and biochemistry of the body can affect human aggression. The limbic system brain area, especially the amygdala, is involved in processing aggressive emotional responses by helping to mediate aggression levels by moderating amounts of testosterone in response to environmental triggers. Research has shown the neurotransmitter serotonin to be involved in aggression, possibly because individuals with low levels of serotonin cannot control aggressive emotional responses. However, some studies also indicate high levels of serotonin to be associated with aggression. Heightened levels of the male hormone testosterone are also associated with high levels of aggression. Testosterone plays a role in moderating aggressive responses in the limbic system, but also affects the activity of serotonin in the brain, which affects aggression levels. No specific ’aggression’ gene has been identified, but studies indicate a genetic influence upon aggression levels. It may be that certain genes moderate testosterone and serotonin production, which then affect aggression levels. The monoamine oxidase A (MAOA) gene, a gene which affects how neurotransmitters, including serotonin, are metabolised, has been dubbed the ’warrior gene’ for its effect upon aggression. It is believed a variation of the gene affects aggression through its sensitivity to social experiences during early development, illustrating the interaction between genetics and environment.
Fig 15.1 H.G. Brunner found evidence of a genetic link to aggression
Brunner (1993) assessed the effect of a shortened version of the MAOA gene on aggression. 5 males from a Dutch family who possessed the shortened version variation of the MAOA gene gave urine samples for analysis. All 5 had a history of aggressive behaviour when they were under threat, frightened, angry or frustrated. They exhibited borderline mental retardation and displayed anti-social behaviours, such as impulsive aggression, arson, exhibitionism and rape. It was found that they all had excessive levels of monoamines (noradrenaline, serotonin and dopamine) in their bodies, which was caused by MAOA deficiency, a condition where individuals do not possess the ability to ’mop up’ excess amounts of neurotransmitters. The findings suggest that the shortened version variation of the MAOA gene can negatively affect aggression levels of male members of families that possess the gene variation through its effect on the biochemistry of the body.
• Raine et al. (1997) scanned the brains of 41 murderers and 41 non-murderers. He found, using PET scans, that some had abnormalities in the way that their limbic systems functioned. This suggests that the limbic system could be implicated in aggressive behaviour.
• Cherek et al. (1996) found that when men take drugs that increase their serotonin levels they display low levels of aggression. This suggests that there may be a causal link between serotonin and aggressive behaviour.
• Huber et al. (1997) found that increasing serotonin levels in animal species increased their aggression levels, casting uncertainty on to whether low or high levels of serotonin cause heightened aggression.
• Higley et al. (1996) found that testosterone affects how aggressive individuals feel, but not whether they will act on that feeling. It seems testosterone levels underpin emotional responses to situations, but that other factors, such as social learning, affect whether the aggression experienced influences the behaviour of individuals.
The MAOA gene may be present in a large number of individuals (about one-third of men in Western cultures possess it) because of the evolutionary advantage it bestows in giving individuals a competitive edge to compete for resources and access to females, which would bring reproductive success.
There is a good degree of research evidence that suggests there are important neural and hormonal influences on aggression levels, especially via the limbic system brain area, the neurotransmitter serotonin, the hormone testosterone and individual genes, such as the MAOA gene — though environmental influences are important too.
The role of the limbic system in aggressive behaviour is not clear cut. The limbic system is made up of many components so it is not altogether clear which parts are implicated. It could equally be that there is an interaction between components of the system.
Much research into neural and hormonal mechanisms in aggression is performed on animals and as such presents problems of generalisation. Aggression appears linked to control of emotional responses, with emotional responses more dependent on cognitive factors in humans than in animals.
Genes cannot account for aggression on their own as genes always need environmental factors to express themselves.
If aggression is seen to be affected by biochemistry this creates the possibility of manufacturing drugs that alter the biochemistry of aggressive individuals to a more reduced and less hostile state.