A liberal outlook may be partly genetic, according to a team of US scientists.
Ideology, they say, is affected not just by social factors, but also by a dopamine receptor gene called DRD4.
The research examined 2,000 subjects from the National Longitudinal Study of Adolescent Health, matching genetic information with maps of the subjects' social networks.
The team found that people with a particular variant of the DRD4 gene were more likely to be liberal as adults - but only if they had an active social life as adolescents.
The finding links up two previous research findings - one showing a link between a variant of this gene and novelty-seeking behavior, and the other a link between novelty-seeking and political liberalism. Previous research has also shown a genetic component to the strength of political views.
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.
This would imply that 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.
"It is the crucial interaction of two factors – the genetic predisposition and the environmental condition of having many friends in adolescence – that is associated with being more liberal," they say. This held true independent of ethnicity, culture, sex or age.
Fowler concludes that the social and institutional environment can't entirely explain a person's political attitudes and beliefs.
"These findings suggest that political affiliation is not based solely on the kind of social environment people experience," said Fowler.
"It is our hope that more scholars will begin to explore the potential interaction of biology and environment. The way forward is to look for replication in different populations and age groups."