Being a liberal may be due to the dopamine receptor gene DRD4, but only if a person had an active social life while a teen, U.S. researchers say. Lead researcher James H. Fowler of University of California, San Diego, and colleagues at Harvard University studies 2,000 subjects from The National Longitudinal Study of Adolescent Health.

They matched genetic information with the study subjects' social networks, and found people with a specific variant of the DRD4 gene were more likely to be liberal as adults -- but only if they had a lot of friends in adolescence. Earlier research identified a link between the gene and novelty-seeking behavior and this behavior has been associated with personality traits related to political liberalism, Fowler says.

"These findings suggest that political affiliation is not based solely on the kind of social environment people experience," Fowler, a professor of political science and medical genetics, says in a statement. People with this genetic predisposition who have a greater-than-average number of friends would be exposed to a wider variety of social factors and lifestyles, which might make them more liberal than average, Fowler says.