Taking a liberal stand in politics could be attributed to the presence of a certain gene, researchers said.
The study from the University of California, San Diego, and Harvard University claimed that ideology is affected not just by social factors, but also by a dopamine receptor gene called DRD4.
By matching genetic information with maps of the subjects' social networks, the researchers were able to show that people with a specific variant of the DRD4 gene were more likely to be liberal as adults, but only if they had an active social life in adolescence.
Lead researcher James H. Fowler of UC San Diego and his colleagues hypothesized that people with the novelty-seeking gene variant would be more interested in learning about their friends' points of view.
So people with this genetic predisposition who have a greater-than-average number of friends would be exposed to a wider variety of social norms and lifestyles, which might make them more liberal than average.
The research team also showed that this held true independent of ethnicity, culture, sex or age.
"These findings suggest that political affiliation is not based solely on the kind of social environment people experience," said Fowler.
"The way forward is to look for replication in different populations and age groups."
The study appears in the latest edition of The Journal of Politics.