Why are some people more likely to engage in violent or criminal behavior than others? New research provides the strongest evidence yet that part of the answer may lie in our DNA.
One gene thought to play a role in violent behavior codes for an enzyme called monoamine oxidase A (MAO-A). After a stressful episode, MAO-A breaks down the neurotransmitters serotonin, norepinephrine, and dopamine so that they no longer exert their effects on the nervous system. The gene for MAO-A has several variants, some of which are associated with relatively higher and lower levels of enzyme activity. In a recent study, researchers showed that males with lower levels were more likely to develop antisocial behavior in response to being maltreated during childhood (Science, 2 August 2002, p. 851.). This may be because they are less able to reduce their neurotransmitter levels to a baseline level after a stressful event, limiting their ability to control aggressive impulses.
Now a team at the National Institute of Mental Health in Bethesda, Maryland has found a more direct link between MAO-A gene variants and violent behavior. The researchers, led by neuroscientist Andreas Meyer-Lindenberg, studied a total of 142 men and women lacking any known history of mental illness or drug or alcohol abuse. The team chose to work with these healthy subjects to minimize the influence of other factors, either genetic or social, that might also alter the brain.
The differences, as revealed by MRI scans, were striking: For example, brain regions known to be involved in control of emotions and impulsivity, such as the anterior cingulate cortex, were on average 8% smaller in subjects with low levels of MAO-A. In addition, when subjects were shown pictures of angry or fearful faces, those with low MAO-A showed much greater activation of the amygdala, a region associated with anger and violent behavior. The team reports its findings online this week in Proceedings of the National Academy of Sciences.
The new study is "an excellent piece of work" says Klaus-Peter Lesch, a neuroscientist at the University of Würzburg in Germany. But neuroscientist Richard Davidson of the University of Wisconsin, Madison, notes that, while the team's use of healthy subjects avoids "confounding factors of psychiatric illness and substance abuse," the relative normalcy of the volunteers prevents the authors from drawing a more direct link between variations in MAO-A and actual aggression.
Meyer-Lindenberg tells ScienceNOW that the team does have unpublished data correlating activation of some brain regions with impulsivity, as revealed by personality tests that some of the subjects have taken. Still, he cautions that variation in the MAO-A gene is only one of many possible risk factors for aggressive behavior. Lesch agrees: "Genes are not destiny."