Researchers found that people born with a common variation of a certain gene had smaller brain regions that manage fear and anger, suggesting they had less ability to control the feelings, scientists reported Monday in the Proceedings of the National Academy of Sciences.
The finding is part of new research beginning to understand the genetics of behavior. The study helps to clarify how a biological predisposition may make some people, especially males, more likely to commit acts of impulsive violence, researchers said.
''This is a first step in a revolution in the field,'' said lead author Andreas Meyer-Lindenberg, a neurologist and psychiatrist at the National Institute of Mental Health in Bethesda, Md., in a telephone interview on Wednesday. ''We are trying to get a handle on brain mechanisms for complex social phenomena.''
The finding has no immediate clinical implications, although it might eventually help lead to new drugs to modify behavior, the scientist said.
''I don't want this to be seen as gene that makes people violent,'' Meyer-Lindenberg said. ''It's naive to expect that you can give people some drug and they would be nonviolent.''
The work centered on the monoamine oxidase A gene, which is associated with impulsive aggression in animals and humans, according to studies of behavior. A 2002 study reported that, among people abused as children, those who carry a particular version of the gene were more likely to victimize other people later in life.
The gene is used by cells to make an enzyme that breaks down serotonin, a chemical that carries signals between brain neurons. In examining 142 people in the study, 57 had a version of the gene that makes less of the enzyme and allows more serotonin to be present during development of the brain.
Brain regions associated with fear were smaller, yet more active, in the people with the version of the gene that makes less of the enzyme. Among those individuals, the males also displayed less ability to inhibit impulsive reactions, according to the study.
Meyer-Lindenberg said it appears likely that having the gene variant affects brain development and helps account for neural mechanisms that may make some people more prone to impulsive violence, as distinct from premeditated violence.
He said behavior affected by the gene is probably caused by the interplay of social and environmental factors, as well as other genetic differences.
While the study suggests that having too much serotonin before birth may make people more prone to anxiety and violence, low serotonin levels in children and adults are associated with depression.
Drugs called MAOIs prevent the breakdown of chemicals including serotonin. Other drugs, called SSRIs are also designed to increase serotonin. Meyer-Lindenberg said new types of medicines might be found to target the brain circuitry affected by the gene variation.