Born to Be Wild?

The role of genes in antisocial behavior

Are criminals born or made? A study published last week in Science suggests they're made, but some people provide better raw material than others.

The study involved the gene that encodes the brain enzyme monoamine oxidase A (MAOA). The MAOA gene is located on the X chromosome, and the enzyme it produces breaks down important neurotransmitters such as serotonin, norepinephrine, and dopamine, rendering them inactive. These neurotransmitters control mood, aggression, and pleasure. Earlier studies have linked genetic deficiencies in MAOA activity to increased aggression in mice and humans. Subjects with low MAOA activity seem to react much more strongly to stress than those with high activity.

In the Science study, researchers at the University of Wisconsin looked at a cohort of 442 males from New Zealand whose lives had been followed from birth to age 26. Genotype analysis found 279 subjects had high MAOA activity and 163 had low activity. The researchers also considered whether the subjects had experienced abuse as children. By age 11, 36 percent had suffered either "severe maltreatment" (8 percent) or "probable maltreatment" (28 percent). Maltreatment was defined as frequent changes in primary caregivers, rejection by the mother, or physical or sexual abuse.

Subjects who both suffered abuse and carried the low-activity MAOA gene were nine times as likely as the rest of the study group to engage in antisocial behavior such as persistent fighting, bullying, lying, stealing, or disobeying rules in adolescence. They accounted for only 12 percent of the subjects but 44 percent of the study group's convictions for violent crime.

The results were even starker for the subjects who had suffered the most serious abuse. "As adults, 85 percent of the severely maltreated children who also had the gene for low MAOA activity developed antisocial outcomes, such as violent criminal behavior," said Terrie Moffitt, one of the lead researchers.

The fact that even in this group some subjects were not antisocial shows that we are talking about higher risks, not predestination. Subjects who carried the low-activity version of the MAOA gene and who were not abused were no more likely to engage in antisocial behavior than those carrying the high-activity version. Conversely, subjects who had been abused but who carried the high-activity version of the MAOA gene were no more likely to engage in antisocial behavior than those who had suffered no abuse at all. "We have identified a gene that seems to protect children from harmful effects of maltreatment," Moffitt told Reuters.

The research focused on males because the MAOA gene is carried on the X chromosome, so males, with only one X chromosome, get only one version of the gene, making its effects easier to see. It is estimated that one-third of all males carry the low-activity version of the gene. The fact that the low-activity variant is so common suggests that it may confer some benefit. What that benefit might be is not known, but perhaps it is associated with a predisposition to taking risks, which could be an advantage in athletes or stock traders.

Females receive two X chromosomes, which means they have a much greater chance of getting at least one high-activity version of the MAOA gene. This may partially explain why women are less prone to violent and criminal behavior. For example, only 15 percent of people arrested in the United States for violent crimes are women.

Interestingly, MAO inhibitors, an important class of antidepressants, boost the presence of neurotransmitters in the brain by preventing MAOA from breaking them down. Although the issue is not addressed by the study, perhaps the flip side of the high-activity MAOA gene is that it predisposes people to depression. Women, in whom high MAOA activity is more prevalent, are twice as likely as men to suffer from depression.

What are the social implications of this study? One can imagine criminal defense attorneys arguing that their clients are not guilty because "their genes made them do it." In our psychotherapy-obsessed culture, this sort of strategy–trying to absolve people of responsibility by attributing their behavior to their genes, their environment, or some combination of the two–is not exactly novel. How often does one hear from death penalty opponents that a murderer must be crazy, since no normal person would commit such a heinous crime?

One can see how this line of argument could progress: Pyromaniacs have irresistible urges to commit arson. Stealing is a disease called kleptomania, whose victims can't help but filch expensive dresses from Rodeo Drive boutiques. Drunk drivers suffer from a genetic predisposition to alcoholism, which means they aren't responsible for ramming a school bus full of children while on a toot.

Clearly, we can go only so far down the slippery slope of genetic absolution. As the Science study shows, environment matters too. Knowing that one will be held responsible for criminal acts inhibits the antisocial impulses that we all harbor from time to time. Just as the balance of brain chemicals can be modified by childhood maltreatment, it can be tempered by social institutions. We punish criminals not just out of vengeance (a much underrated motive, in my opinion) but also to deter others. Altering behavior ultimately means altering brain chemicals, so in a real sense most social institutions aim to maximize prosocial brain chemicals. Given the fact that most of us don't commit violent crimes, social pressures seem to have been pretty successful at accomplishing that goal.

Even if social institutions can't prevent all crimes, they can isolate criminals. In order for most people to live peaceably, those who succumb to whatever predispositions to violence they may have simply cannot be allowed to prey upon the rest of us with the excuse that they can't help themselves. If they really can't help themselves, they must be put away where they can't harm us.

Looking further down the road, the Science study notes that "these findings could inform the development of future pharmacological treatments." In other words, it may someday be possible to offer those with low MAOA activity a drug to calm their violent impulses. Such treatment won't necessarily be coercive. Just as people who suffer from depression ask for antidepressants to control their brain chemistry, it is likely that many people who know they are at risk of violent outbursts will ask for drugs to boost their MAOA activity.

In the meantime, a clear message of the study is that one of the best ways to prevent future criminal behavior is to prevent child abuse now. That's surely a worthy social goal.