Die-hard liberals and conservatives aren’t made, they’re born. It’s literally in their DNA.

That’s the suggestion of a study by a group of researchers who wanted to see whether there was a biological basis for peoples’ political attitudes.

They found to their surprise that opinions on contentious issues such as gun control, pacifism and capital punishment are strongly associated with physiological traits that are probably present at birth.

The key is the different levels of fear that people naturally feel.

“What is revolutionary about this paper is that it shows the path from genes to physiology to behavior,” said James H. Fowler, a political science professor at the University of California, San Diego, who was not involved in the research.

The researchers, whose findings were published today in the journal Science, looked at 46 people who fell into two camps: liberals who supported foreign aid, immigration, pacifism and gun control; and conservatives who advocated defense spending, capital punishment, patriotism and the Iraq war.

In an initial experiment, subjects were shown a series of images that included a bloody face, maggots in a wound and a spider on a frightened face. A device measured the electrical conductance of their skin, a physiological reaction that indicates fear.

In a second experiment, researchers measured eye blinks – another indicator of fear – as subjects responded to sudden blasts of noise.

Compared to staunch liberals, people with strongly conservative views were three times more fearful after factoring out the effects of gender, age, income and education, all of which can affect political attitudes.

Fowler said the study added to the growing research suggesting that over millions of years, humans have developed two cognitive styles: conservative and liberal. Cautious conservatives prevented societies from taking undue risks, while more flexible liberals fostered cooperation.

“For the species to survive, you need both,” he said.

But Jon A. Krosnick, a political science professor at Stanford University, said it was impossible to draw any conclusions from a study with so few people.

“I don’t believe any of this,” he said. “The people who are most scared are less in favor of gun control. Why wouldn’t they be more in favor? Because they need guns to fight the bad guys? You can make up a story in either direction.”