Study Links Antisocial Behavior to Abuse, Variant Gene

Men who were mistreated during childhood are more likely to become criminals or exhibit other antisocial behavior if they have a single gene variation that results in low activity of an important brain enzyme, according to a study published today.

Researchers studied 442 men who were part of a larger New Zealand study that followed the men from their birth in 1972. Out of the group, 154 were mistreated as children--a condition the researchers defined as having been rejected by their mothers, physically or sexually abused, or subjected to frequent changes in their primary caregivers.

The researchers found that 85% of the abused children who had the gene variation were eventually involved in some form of antisocial behavior as adults, including violent crime.

But researchers cautioned that the gene variation cannot be used to predict who will become violent. The study found that men who had the low-activity form of the gene but were not abused as children showed no increased risk of behavioral problems.

Terrie Moffitt, one of the study’s researchers and a professor of psychiatry at King’s College London and the University of Wisconsin, said the two findings underscore the interaction between genes and environment in influencing human behavior. “People shouldn’t think they have no choice of whether to be violent or not,” she said.

Moffitt’s research focused on a gene called monoamine oxidase A, or MAOA. The gene controls the production of an enzyme that breaks down neurotransmitters, such as serotonin, norepinephrine and dopamine.

There are several variations of the gene. The most common, found in about two-thirds of the male population, is a high-activity version that produces more of the MAOA enzyme. The low-activity variation is found in about one-third of males.

Scientists have also identified a third variation that results in the complete absence of the MAOA enzyme. It was found almost 10 years ago in a Dutch family plagued by several violent members. This form is extremely rare and hasn’t been identified in any other families.

The function of the MAOA enzyme is to clear neurotransmitters from the brain’s signaling system. Moffitt said no one knows specifically how the enzyme affects behavior, but it may act as a reset button in stressful situations, she said.

The study found that children who were abused and had the low-activity gene were more than three times more likely to be antisocial than their high-activity counterparts.

Scientists evaluated adult behavior by looking at criminal records, psychiatric evaluations and a personality questionnaire. The research looked at men because women are much less likely to inherit only the low-activity gene.

Moffitt said the predictive ability of the low-activity gene for antisocial behavior in abused children is far from certain.

Moffitt emphasized that the study also found that having the high-activity gene could be protective in children who have experienced severely stressful situations. About 25% of children who were abused but had the high-activity gene were eventually involved in antisocial behavior, far lower than the approximately 50% rate for abused children as a whole.

“No one knows why kids with the protective genotype are more resistant to trauma,” Moffitt said. But she added: “If genes have been kept in the population for hundreds of years, they are probably conferring an advantage.”

Laura Baker, an associate professor of psychology at USC, commended the study as one of the first to pinpoint a gene that can modify a person’s vulnerability to environmental stresses.

But Evan Balaban, director of the neurosciences program at City University of New York, said the results of the study are hard to interpret. They are missing many of the steps that link gene activity with the behavior being studied, Balaban said.

Balaban has criticized previous research on the biological basis of violent behavior and said that many people classified as having violent or aggressive characteristics were mentally ill.

He said this is also a possibility in this study: The low-activity gene may be associated with mental illness, which can lead to more inappropriate behavior in general.