A new study finds that aggressive men who were mistreated as children were also more likely to have low levels of a particular brain chemical. The research, published in the current issue of the journal "Science," is the latest to implicate the brain chemical MAOA in aggressive behavior.

Psychologists have long known that many abused children grow up to be angry, abusive adults. But some don't. The question has always been what makes some people more resilient than others. The answer may have to do with a brain chemical, or enzyme, called MAOA.

Terrie Moffett is a psychology professor and researcher at the University of Wisconsin in Madison and at Kings College in London. "The enzyme's job in the brain is to recycle excess neurotransmitters after they are used and clear them away so that the brain maintains a healthy balance of neurotransmitters like serotonin and dopamine and other neurotransmitters as well."

Neurotransmitters, such as serotonin and dopamine, are chemicals brain cells use to communicate with one another. Low levels of serotonin are linked to depression. Too little dopamine causes Parkinson's disease.

Studies in mice show that an abnormality in the gene that produces MAOA makes animals with little or none of the enzyme extremely jittery and aggressive. The same thing was found in a human study of MAOA involving a large Dutch family. Male members with low levels became extremely aggressive when angry, fearful or frustrated.

In the latest study, Professor Moffett and colleagues wanted to see whether individuals who produced a low amount of MAOA were more prone to aggressiveness than those with normal levels. But not only that. The researchers wanted to find out what role environment played in making those with low MAOA anti-social adults.

The investigators followed 442 male children born in a New Zealand town in 1972 for 26 years. The researchers determined which of the participants produced normal levels of MAOA and which produced low levels of the enzyme from blood tests.

The researchers then compared that information to data on how the men were treated as young children, including whether they were rejected by their mothers, had multiple caregivers or were physically or sexually abused, and whether, years later, they were prone to aggressiveness or had gotten into trouble with the law.

Not surprisingly, Professor Moffett says investigators found that the people who were maltreated were more likely to be the same people who were violent. "But that depended on their genetic make-up. I suppose the best way to describe the finding is to say that overall about half of children who are severely maltreated tend to become engaged in antisocial behavior when they grow up," he said. "If they have the risky genotype, then that increased the risk to 85 percent. So, it really exacerbated the effect of maltreatment - the harm that was done to the children."

On the positive side, researchers found the MAOA gene appears to be protective. Among participants who had the gene that produced normal amounts of MAOA, less than one-fourth became anti-social adults.

Jean Shih is a toxicologist at the University of Southern California who has studied MAOA for 30 years. Professor Shih's work has involved genetically-altered animals that produce no MAOA. "We knock out totally the MAO activity. And this tells us that neurotransmitters play a role, but by no means it's the only factor. So that's why it's very interesting that this paper reported that environment is very important," she says. "Actually, we know the environment factor is important also because these animals - they could be normal but only if you put them in a mild stress, they will behave differently."

While the study of New Zealand boys identified those with low MAOA who became disruptive and aggressive adults, researcher Terrie Moffett says she prefers to focus on the gene's protective effect. "If you think about it... genes have got a bad reputation with the general public because the general public is constantly learning that their genes are lurking inside them, ready to cause cancer or cause them to develop Alzheimer's disease or cause them to become schizophrenic or some terrible outcome like that," she says. "And the interesting thing about this is we seem to have found a genotype that may actually help protect people against the stress and difficulties of life."

Professor Moffett is also interested in MAOA research involving women.