Discuss genetic factors in aggressive behaviour. (8 marks + 16 marks)

Here you should outline and evaluate the role of genetic factors in aggression. It can be difficult to distinguish between AO1 and AO2 content in this question, so when you want to use a research study as AO2 material be sure to make this clear by using a lead-in sentence such as 'There is research evidence to support this'.


The role of genetic factors in aggression can be tested in twin studies. Genes are identical in sets of monozygotic twins but are different between sets of dizygotic twins, meaning that if monozygotic twins are more alike in terms of aggression than dizygotic twins are, this similarity can be attributed to genetics. These twin studies have generally found that almost 50% of variance in aggressive behaviour can be attributed to genetic factors.

Adoption studies can help determine the relative contributions of environment and heredity in aggression. Positive correlation between adopted children and their biological parents implies a genetic effect, while positive correlation between the child and their adoptive parents implies an environmental effect. A study of over 14,000 adoptions in Denmark found that a significant number of adopted boys with criminal convictions had biological fathers with criminal convictions, demonstrating a genetic effect.

One gene which has been linked to aggression is the gene coding for the production of monoamine oxidise A (MAOA). MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with aggressive behaviour. Thus, low levels of MAOA are also associated with aggression. Studies of violent criminals have found that they often have a defect in the gene that produces MAOA.

Gene variants can also affect the production of testosterone, a hormone which increases levels of aggression. Sex hormone-binding globulin (SHBG) is a glycoprotein that binds to hormones such as testosterone, but a common variant in the gene coding for the production in SHBG can result in lower levels of SHBG being produced. This means that there are higher concentrations of unbinded testosterone, causing the body produces less testosterone. Thus, inheriting these gene variants can decrease levels of aggression.

Although there is a relationship between genes and aggression, it is very difficult to determine the nature of this relationship. This is because many different genes contribute to aggression in various ways. The net effect of genes is extremely complex, and we are only aware of the influence of a few genes (such as the gene coding for MAOA).

This explanation has also been criticised for being too reductionist. As well as genetic factors, there are many environmental factors that influence the manifestation of aggressive behaviour. Genetic factors can also influence environmental factors and vice versa in gene-environment interactions. This explanation only focuses on genes, though genes and the environment are intrinsically related: one cannot be fully explained without the other.

Most of the studies mentioned have measured aggression using self-report techniques or observational studies. However, studies using self-report techniques found a greater genetic influence on aggression, while studies using observation found a greater environmental influence. This suggests problems with the methodology of many studies: if these studies were valid, one would expect no differences between the results of self-report studies and those of observational studies.

MAOA is thought to increase aggressive behaviour by affecting levels of neurotransmitters. Research support for this comes from Cases et al., who disabled the MAOA gene in mice. They found that in these mice, levels of serotonin and dopamine (both of which are associated with higher levels of aggression) increased and aggression increased. This suggests that MAOA reduces aggression by regulating levels of these neurotransmitters.

Studies such this one on non-human animals have an important role in helping us understand aggression in humans, as genes can be manipulated and the effects of these manipulations can be observed. This allows us to precisely identify the role of specific genes. However, commonly-used animals such as mice have a different genome and physiology to humans, so the effects of genes may be different. The use of animals in such studies can also be considered unethical.

Many of the studies in this area have focused on individuals convicted of violent crime. However, these results only represent a small minority of those who are regularly involved in aggressive behaviour, as many violent attacks do not results in a conviction. These individuals may also not be habitual offenders; they may be usually calm people who were designated as violent for a one-time offense. This may explain why so many studies have found little or no evidence of heritability for violence.

This research has an important real-world application.  There have been suggestions that if people's genes predispose them towards aggressive behaviour, genetic engineering should be used to change their genes and reduce this risk. Some have even suggested more extreme measures to prevent the heritability of such genes, for example by chemical castration. However the labelling of an individual as dangerous based on their genetic inheritance poses serious ethical questions.

Explanations that are purely genetic have been criticised for being too deterministic. They argue that our aggression is pre-programmed, while ignoring the human characteristic of free will. If aggression is purely biologically determined, people cannot be held responsible for their actions; this may have further implications for the legal system when dealing with acts of aggression.