Yes, at least in part. The latest evidence is in a study published in the Journal of Neuroscience.

Stanford University researchers looked at 14 young adults with a genetic condition called Williams syndrome and 13 without the syndrome. People with Williams syndrome generally have poor visual-spatial skills – they would have trouble, for example, drawing a bike – but are abnormally good at recognizing faces and are often far more extroverted than normal.

The syndrome is caused by the deletion of genes on one chromosome and is not inherited, says Dr. Barbara Pober, a Massachusetts General Hospital geneticist who studies the condition.

In the Stanford study, researchers asked all participants to look at pictures of happy and of fearful faces while their brains were being scanned by MRI machines and while they had electrodes on their scalps to detect bursts of brain activity, particularly in a small area called the amygdala. The amygdala provides instant emotional reactions to stimuli, before the higher brain centers more carefully assess danger or safety, friend or foe.

When shown pictures of happy faces, the people with Williams syndrome showed greater amygdala activity than those without, which fits with the longstanding observation of abnormally vigorous sociability, says Brian Haas, the lead author of the paper. The amygdalas of people with Williams syndrome also responded less vigorously to fearful faces, which also may account for their greater extroversion and less fear of people.

This new finding on gregariousness fits with recently published research on twins conducted by researchers from Harvard University and the University of California, San Diego, on the genetic underpinnings of the propensity to form social networks. Identical twins, whose genes are the same, were more likely to show the same pattern of social networks than fraternal twins, who share only 50 percent of genes.

“Most people think that personality style and preferences are totally under our control, but that’s not true,” says Pober. “Findings from Williams syndrome show us that changes in genes can have dramatic effects on personality traits.”