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Researchers at the National Institute of Mental Health (NIMH) have discovered a genetically controlled brain mechanism responsible for social behavior in humans ? one of the most important but least understood aspects of human nature.

The study compared the brains of healthy volunteers to those with a genetic abnormality, Williams Syndrome, a rare disorder that causes unique changes in social behavior. This comparison enabled the researchers to both define a brain circuit for social function in the healthy human brain, and identify the specific way in which it was affected by genetic changes in Williams Syndrome.

People with Williams Syndrome, who are missing about 21 genes on chromosome seven, are highly social and empathetic, even in situations that would elicit fear and anxiety in healthy people. They will eagerly, and often impulsively, engage in social interactions, even with strangers.

However, they experience increased anxiety that is non-social, such as fear of spiders or heights (phobias) and worry excessively.

For several years, scientists have suspected that abnormal processing in the amygdala, an almond-shaped structure deep in the brain, may be involved in this striking pattern of behavior. The amygdala's response and regulation are thought to be critical to people's social behavior through the monitoring of daily life events such as danger signals. Scientists know from animal studies that damage to the amygdala impairs social functioning.

"Social interactions are central to human experience and well?being, and are adversely affected in psychiatric illness. This may be the first study to identify functional disturbances in a brain pathway associated with abnormal social behavior caused by a genetic disorder."

The scientists looked at the whole brain to identify other regions where reactivity was different between Williams participants and healthy volunteers. They identified three areas of the prefrontal cortex, located in the front part of the brain, that have been implicated in decision-making, representation of social knowledge, and judgment. Those regions are the dorsolateral, the medial, and the orbitofrontal cortex.

Specifically, the dorsolateral area is thought to establish and maintain social goals governing an interaction; the medial area has been associated with empathy and regulation of negative emotion; and orbitofrontal region is involved in assigning emotional values to a situation.

The researchers found a delicate network by which these three regions modulate amygdala activity. In Williams Syndrome, this fragile system was significantly abnormal, particularly the orbitofrontal cortex. This area did not activate for either task and was not functionally linked to the amygdala, as it was in healthy controls.

Instead, the scientists observed increased activity and linkage in the medial region, which is consistent with the high level of empathy exhibited by people with Williams Syndrome.