One of the ways that animals are used in genetic research is through selective breeding experiments. This is when animals are bred with the goal of producing a specific phenotype. 

Modern research using selective breeding often uses transgenic mice – that is, a mouse that has had a single gene changed or removed. In 2007 Mario Capecchi, Oliver Smithies, and Martin Evans won the Nobel Prize in Physiology for developing this procedure. These mice are also sometimes referred to as “knockout mice.”

Cases et al (1995) carried out a study on the genetic origins of aggression. For their study, they used a transgenic mouse where the gene that regulates the production of monoamine oxidase A (MAOA), an enzyme that breaks down serotonin and norepinephrine, was ‘knocked out” or deleted. High levels of serotonin and norepinephrine were found in the offspring of the transgenic mice.  High levels of aggression were found in the male pups.

Does this animal model then help us to understand human behaviour?  In a study carried out by Caspi et al (2002 ), the researchers carried out a 26-year longitudinal study of 1037 children born in Dunedin, New Zealand.  The study included 442 boys. In their study, they looked at the genotype of the boys – particularly at the gene that controls the production of the MAOA enzyme – the same enzyme studied by Cases et al (1995).

The researchers wanted to see not only if the gene had an effect on the level of aggression in the children as they developed, but also whether environmental stressors may interact with the gene to determine behaviour.  In other words, they were interested in the gene-environment interaction that is important to epigenetics.

By age 11, 36% of the children had been maltreated; this included rejection by the mother, physical and/or sexual abuse.  The results of the study showed that if the abused boy had the version of the MAOA gene that resulted in low levels of enzyme production, they were more likely to bully others and engage in aggressive and antisocial behaviour. As adults, 85 percent of the severely maltreated children who also had the gene for low MAOA activity developed anti-social outcomes, such as violent criminal behaviour.  Boys who were abused but did not have this gene did not show any more aggression than boys that were not abused.

It appears that this study confirms the findings in the animal model, thus bringing support to the theory of genetic influence in aggressive behaviour.