The 'liberal gene' is a "transmitter" in the brain called DRD4 connected to the chemical dopamine, known as the reward currency of the brain.

When those with the gene are socially outgoing, they seek out and are "rewarded" by other people's points of view, far more than they might be without the gene.

This suggests they are more open-minded and tend to form less conventional political viewpoints as adults -- especially if they have a socially active adolescence, The Telegraph said about the study.

Scientists from the University of California and Harvard in the US looked at 2,000 Americans, according to the Journal of Politics.

It found that those with a strain of the DRD4 gene seek out "novelty" -- such as people and lifestyles that are different to the ones they are used to. This leads them to have more liberal opinions politically, it found.

The person's age, ethnicity, gender or culture appeared to make no difference -- it was the gene that counted.

DRD4 is controlled by dopamine, which affects the way the brain deals with emotions, pleasure and pain and can, therefore, influence personality traits.

The more popular someone with this gene is in their young formative years the more likely they are to hold liberal beliefs.

James Fowler, the professor who led the study, said: "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.