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Two strikes. Boys with a certain version of a gene for a brain enzyme--and who are abused--are more likely to be violent.

Violent Effects of Abuse Tied to Gene

Some children who suffer physical, sexual, or emotional abuse become violent adults. But many do not. Now a study of both genetics and social surroundings points to the influence of a particular gene on aggressive behavior in young adults from troubled backgrounds.

The gene codes for an enzyme called monoamine oxidase A (MAOA), which metabolizes several kinds of neurotransmitters in the brain. Studies of lab animals show that knocking out the MAOA gene makes adult mice more aggressive. The first suggested evidence in humans came from a 1993 report of a Dutch family (Science, 18 June 1993, p. 1722). Several men in this family had a defective MAOA gene--none of the enzyme was found in their cerebrospinal fluid--and were prone to impulsive bouts of aggression.

To see whether the MAOA gene influences aggressive behavior in the broader population, a team led by clinical psychologists Terrie Moffitt and Avshalom Caspi of King's College London and the University of Wisconsin, Madison, turned to a New Zealand study, begun in 1972, that has followed a group of children since birth. Of 442 selected boys, those who were severely maltreated were more likely to exhibit so-called antisocial behavior than boys who had suffered little or no abuse. But the researchers also found that antisocial behavior was more likely in males with the genotype for low MAOA activity who had been mistreated. The 55 boys in this group were about twice as likely to have been diagnosed with conduct disorder in adolescence as the 99 mistreated boys with the high-activity genotype. And they were 10 times more likely to be convicted of a violent crime by age 26, the researchers report in the 2 August issue of Science. In the absence of abuse, having the low-activity genotype didn't make boys any more likely to be antisocial.

"This is a very important piece of work," says geneticist Greg Carey of the University of Colorado, Boulder. "It's pretty convincing for just a single study." Jon Beckwith of Harvard Medical School in Boston adds, "I would use this as a wonderful class example of how social factors can play an enormous role in expression of behavioral traits."