Early research concentrated on chromosomal abnormalities but no specific genotype has been associated with aggression. Genes determine how much testosterone a person’s body produces and how quickly it circulates around their body. Genes also determine the synthesis of testosterone receptors, and how many and how sensitive such receptors are. The gene called monoamine oxidase A (MAOA) produces an enzyme which regulates the metabolism of serotonin in the brain. Aggressiveness is influenced by a variation in the MAOA gene; Low level activity of MAOA results in low levels of serotonin which have been associated with increased risk of aggression but the high MAOA variation is not.
A strength of this explanation is that it is supported by research. Cases et al found that mice genetically engineered to lack MAOA had a dramatically altered serotonin metabolism and as adults showed enhances aggression particularly during mating, suggesting that aggression is a direct result of MAOA deficiency rather than other genetic influences or psychosocial factors. Further research to support this comes from Brunner, who found that in a large Dutch family where all the males had a mutant form of the MAOA gene, they all acted aggressively when angry or fearful, suggesting this low variant resulted in low levels of Serotonin which increased their aggressive responses. However a limitation of this kind of research is that there are many qualitative and quantitative differences between mice and humans so making it difficult to generalise the findings of this kind of research directly to humans. Despite this there are practical advantages to using animals like mice such as quicker breeding cycles and some researchers would argue it is more ethical to research aggression on contained animals than on humans, others of course disagree.
Further research into MAOA has found that on its own, the low MAOA gene variant, has no effect. It seems aggressiveness is influenced by a variation in the MAOA gene which is sensitive to negative social experiences early in human development. Further research to support this development comes from Moffitt et al who found in a longitudinal study on New Zealand males that those who had suffered abuse as a child and had the low MAOA variation were nine times more likely to display aggressive behaviour than those who had been abused but carried the high MAOA variant or those who had not been abused. This indicates that an interaction of genetics and environment is at work in determining human aggression. A strength of the low MAOA variation is that theoretically it may be possible to devise drug treatments for people with this variation to help control aggressive urges. However research by Tyler reported that drug treatments were not having any success, suggesting that other non-biological factors may be involved. Furthermore, the purely genetic basis of aggression is a limited explanation because it has proved elusive and it would be reductionist to argue this because it appears that numerous genes are involved. These interact with environmental stimuli to cause aggression which is an interactionist view.