Don't hold liberals responsible for their opinion -- they can't help themselves.

A new study has concluded that ideology is not just a social thing; it's built into the DNA, borne along by a gene called DRD4. Tagged "the liberal gene," DRD4 is the first specific bit of human DNA that predisposes people to certain political views, the study's authors claim.

And the key to it all: Liberals are more open, said lead researcher James H. Fowler, a professor of both medical genetics and political science at the University of California, San Diego.

"The way openness is measured, it's really about receptivity to different lifestyles, for example, or different norms or customs," he told FoxNews.com. "We hypothesize that individuals with a genetic predisposition toward seeking out new experiences [a measure of openness] will tend to be more liberal" -- but only if they had a number of friends when growing up, Fowler cautioned.

This isn't a typical gene association study," he said. "There's a combination of genes and environment that matter."

The paper, which appears in the latest edition of The Journal of Politics, focused on 2,000 subjects from The National Longitudinal Study of Adolescent Health. By matching genetic information with maps of each individual's social network, the researchers were able to show that people with a specific variant of the DRD4 gene were more likely to be liberal as adults -- though only if they had an active adolescent social life.

The research, sponsored by the National Science Foundation, focused on dopamine -- a neurotransmitter that affects a wide variety of brain processes, including control over movement, emotions, and ability to experience pleasure and pain. Previous research identified connections between a variant of this gene and novelty-seeking behavior, a personality trait that numerous studies have shown is linked to political liberalism, the authors say.

So if a liberal ideology is genetic, or at least partly genetic, does that mean it runs in the family -- and can be passed on to your kids? No doubt about it, the authors say.

"In fact, psychologists have asserted for many years that social conservatism is heritable," the paper notes. Fowler cites twin studies comparing fraternal and identical twins that have pinpointed exactly how much liberal ideology can be passed along.

"Ideology is about 40 percent heritable. It's almost half genes and half environment," Fowler told FoxNews.com. What's more, he said, any trait that can be inherited has potentially been with the human race for a long time, meaning political ideology has been a part of us for tens of millions of years.

"If it's really the case that genetic variation is influencing ideology, this isn't something we've been living with for the last ten years," he told FoxNews.com. "These are processes that have been going on for the past million years."

Fowler suggests that it made more sense to be liberal in certain environments at specific points in human history, and in others a conservative ideology was merited. "And this is what made it possible for our species to survive," he said.

"If it made sense for us all to be liberal, natural selection would have made us all liberal."

Fowler and colleagues Jaime E. Settle and Christoper T. Dawkes, also of UC San Diego, and Harvard University's Nicholas A. Christakis hypothesized that people with the novelty-seeking gene variant would be more interested in learning about their friends' points of view. As a consequence, 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, they deduced.

The researchers reported that "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."

The research team also showed that this held true independent of ethnicity, culture, sex or age.