A brain primed for violence?
By Helen Phillips PEOPLE with a gene variant known to be linked to aggression may have been born with key brain differences that could make them more likely to snap under pressure. The gene, called MAOA, produces the enzyme monoamine oxidase-A. Complete absence of this gene, though rare in humans, has been linked to aggressive behaviour in men, and mice engineered to lack MAOA are also unusually aggressive. Many more people, however, carry a low-activity variant of the gene, known as MAOA-L. A study in 2002 found that men with MAOA-L who had been maltreated as children were more likely to exhibit antisocial behaviour than those with a similar background who had the normal MAOA gene. Now psychiatrist Andreas Meyer-Lindenberg and colleagues from the US National Institute of Mental Health in Bethesda, Maryland, have discovered differences in brain structure and function that might underlie this link. They looked at the genes of 142 healthy men and women with no history of criminality, violence or abuse, and found that 57 had the MAOA-L variant. Brain scans of the same group revealed that the amygdala and cingulate cortex, which are involved in the perception and regulation of emotion, were on average significantly smaller in men and women with the L variant. There were some differences in brain activity too. When the researchers showed the volunteers frightening images, the amygdala appeared to overreact in those people who had the L variant. And in the men with MAOA-L, regions that normally regulate the amygdala response, including the cingulate cortex and parts of the prefrontal cortex, were underactive (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0511311103). Thus while both sexes may have heightened emotional responses, men with MAOA-L “are less able to inhibit their responses”, says Meyer-Lindenberg. Men with the gene variant that is linked to aggression may be less able to inhibit their emotional responses To some extent, the differences between the sexes may arise because the MAOA gene lies on the X chromosome, so men have only one copy. Women have two copies of the gene, and so are likely to have higher levels of the enzyme monoamine oxidase-A. The differences in brain structure may become established before birth. Low levels of the enzyme have been linked to high levels of brain signalling chemicals in the fetus, including serotonin, which might affect developing brain circuits. However, Meyer-Lindenberg is careful to warn against using MAOA-L as a predictor of whether someone is likely to become violent. Many other genes may be involved, not to mention social and environmental factors. “There is certainly not enough evidence to feel that a person who has a combination of risks should be weighed differently in a legal sense,” he says. Knowing whether someone is less likely to be able to control their emotional responses could, however, have enormous potential for tailoring drug or behavioural treatments for people who suffer trauma at an early age, says Essi Viding of University College London,