BEAUTY opens many doors. Study after study has concluded that the comely earn more, are better liked, are treated more indulgently and are even given more lenient sentences in court than their plainer counterparts. The door it opens widest, though, is the romantic one. As both common sense and evolutionary theory suggest should happen, beautiful people attract beautiful partners. But not always. Occasionally, handsome men choose plain women, and vice versa.

Why this should be vexes psychologists and biologists alike. A study by Lucy Hunt of the University of Texas at Austin, and her colleagues, soon to be published in Psychological Science, suggests an answer. It depends on whether the couples in questions were friends before they were lovers.

Ms Hunt asked 167 couples how they had come to know one another. Individuals were questioned alone, and their responses compared with those of their other halves. Those responses were mixed: 40% of couples said they were friends before they were lovers, while 41% said they were not. (The remaining 19% could not agree, with one member of the couple saying they had been friends beforehand and the other denying it.)

To measure participants’ attractiveness, Ms Hunt showed videos of them to a group of undergraduates who had not been told the experiment’s goal. The students were asked to rate the attractiveness of the people in the videos on a seven-point scale. Generally, the scores assigned to a participant by different students agreed with one another, which allowed Ms Hunt to calculate, with a fair degree of confidence, just how well-correlated in the beauty stakes a pair of lovers were.

She and her colleagues found that the attractiveness of couples who became romantic partners soon after their first meeting had an average correlation of 0.46 (out of a maximum possible of 1.0). In other words, if a man in such a couple was rated as “very attractive”, there was a fair chance that his female counterpart would be rated as “very attractive” too. In contrast, those who were friends first had an average correlation of just 0.18. Tellingly, the researchers found that the longer the members of a couple had known each other before becoming lovers, the lower was the strength of the correlation.

So a period of pre-romantic friendship can indeed erode beauty’s pulling power. But why? One explanation Ms Hunt proposes is that friendship gives potential mates time to assess subtler attributes, such as intelligence and dependability, as well as the more obvious signal of outward beauty. Given the huge commitment, by both sexes, involved in raising children, such a strategy of long-term assessment is likely to have evolution on its side.

That does, though, raise questions. One is, why does love at first sight persist? Another is, if beauty is, in an evolutionary sense, tradable for good parenting skills, what does that have to say about the parenting skills of beautiful couples?