RANDY THORNHILL is not a man to shy away from controversy. It was he who showed, just over a decade ago, that like most other animals who pick their mates by sight, the thing that humans seem to value above all in a partner is bodily symmetry. It was also he who argued that far from being a pathology, a propensity to rape is a normal part of the evolutionary inheritance of men—a point of view previously confined to militant feminism. Now he is looking at the evolutionary roots of political preference. In particular, he is asking if there is a Darwinian explanation for why some people are liberals and some conservatives. Curiously, his answer also casts light on the ideas of another 19th-century scientist, Sigmund Freud.

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Psychologists have long known that conservatives and liberals differ in more than whether they vote Tory or Labour, Republican or Democrat. Indeed, the perverse mixtures of policies on offer from these parties, conflating social conservatism with economic liberalism and vice versa, mean that voting preferences are often an unreliable guide. But bundles of personality traits do tend to cluster together in people. Some are sceptical of tradition, open to new experiences, rebellious, pleasure-seeking, egalitarian and risk-prone. Others value tradition, duty, close family relationships and security.

What no one knows is what causes the difference—and, inevitably, some people prefer genetic explanations while others cleave to environmental ones. Dr Thornhill and his colleague, Corey Fincher, however, come down on neither of these sides. They think an individual's early environment is responsible for switching on one pattern or the other, but that this switch is an evolved—and thus genetically controlled—response.

The crucial environmental factor, they believe, is childhood stress—particularly how securely someone was attached to his mother when a child. And they lay out their case that this is so in a paper just published in Evolution and Human Behavior.

To conduct their study, the two researchers asked 123 students from the University of New Mexico, where they both work, to fill in a questionnaire. This asked about the volunteers' political beliefs, personality traits and childhood memories. Dr Thornhill and Dr Fincher then used a version of what is known as the Wilson-Patterson conservatism scale to classify their volunteers as conservatives or liberals on the basis of their personality traits. (Conservatives complain, with some justification in this case, that academics have a tendency to pathologise conservatism by defining liberalism as a default from which conservative tendencies deviate, in the same way that a schizophrenia scale or a depressiveness scale measures a deviation from healthy normality.)

When they correlated their volunteers' Wilson-Patterson scores with childhood memories the researchers discovered that those classified as conservatives tended to report having had secure childhoods with strong attachments to one or both parents, and low stress. Those classified as liberals reported having stressful childhoods and weaker attachments to their parents. Freud, on this reading, was right about the importance of early experience on character formation.

The second part of the paper is more speculative. Conservatives, the authors suggest, are likely to do best in stable societies—say, a village where there is plenty to eat and no marauding enemy, and the biggest danger to stability might come from dissatisfaction within the community. The traits associated with liberalism, on the other hand, might be most effective when there is a lot of stress on a population. If food is scarce and other people are pressing in, then a liberal's willingness to challenge traditional ways of doing things, and his greater willingness to embrace outsiders (often literally; liberals tend to have more sexual encounters than conservatives) might come in handy. Evolution has thus fixed it so that appropriate patterns of personality are activated in response to the environment in which they are most likely to cause an individual to thrive.

Which is all fine and dandy—except that last year a group of researchers at the University of California, Berkeley, came to exactly the opposite conclusion. Their study found that insecure and fearful children were more likely to grow up into conservatives, and that confident kids were more likely to become liberal. Clearly, as scientists are so fond of saying, more research is needed.