Could the U.C.L.A. researchers be creating the political science of the future? Consider this possibility: the scientists do an exhaustive survey and it turns out that liberal brains have, on average, more active amygdalas than conservative ones. It's a plausible outcome that matches some of our stereotypes about liberal values: an aversion to human suffering, an unwillingness to rationalize capital punishment and military force, a fondness for candidates who like to feel our pain.

What would that kind of insight tell us that we didn't know already? One thing is certain: evidence of a neurological difference between liberal and conservative brains would not be another instance of genetic determinism, since patterns of brain activity are shaped by experience as much as by genes. (Those who suffer from post-traumatic stress syndrome also show unusual patterns of amygdala activity, but those patterns are almost inevitably the imprint of a specific event, and not the long arm of DNA.)

Nonetheless, opening up the brain's black box might provide new explanations for how people become Republicans or Democrats, not to mention libertarians or Maoists, in the first place. It's pretty to think that we all decide our political affiliations by methodically studying each party's positions on the issues. But a recent study by Paul Goren at Arizona State found that voters typically formed their party affiliations before developing specific political values. They become Democrats first and then decide that they, say, oppose capital punishment and support trade unions. But how do they make that initial decision to be a Democrat? The most likely indicator of political preference is your parents' party affiliation, but if everyone simply voted along family lines, the dominant party would simply be the one whose members had the most voting offspring. The real question is why someone would ever break from the family tradition -- without feeling strongly either way about specific issues.

Those M.R.I. scans suggest an explanation. Perhaps we form political affiliations by semiconsciously detecting commonalities with other people, commonalities that ultimately reflect a shared pattern of brain function. In the mid-1960's, the social psychologist Donn Byrne conducted a series of experiments in which the participants were given a description of several hypothetical strangers' attitudes and beliefs. They were then asked which stranger they would most enjoy having as a co-worker. The subjects consistently preferred the company of strangers with attitudes similar to their own. Opposites repel.

Say you're inclined to form strong emotional responses to images of violence or human suffering, and over the course of your formative years, most of the people you meet who respond to these images with comparable affect turn out to be Democrats. That's a commonality of experience that exists beneath conscious political affiliation -- it's closer to a gut instinct than a rational choice -- but if you meet enough Democrats who share that experience, sooner or later you start carrying the card yourself. Political identity starts with a shared temperament and only afterward deposits a layer of positions on the issues.