OKAY girls. 'Fess up. Do you prefer rugged, testosterone-fuelled good looks in a man, or more sensitive facial features? And why? A thousand women's magazines have blazed a trail in asking that question, but evolutionary biologists are now treading cautiously in their wake.

The latest group to tiptoe into the minefield is led by Lisa DeBruine, of the University of St Andrews, in Scotland. She and her colleagues have attempted to reconcile the sometimes conflicting results obtained by previous researchers in a paper just published in the Proceedings of the Royal Society.

It is well established that facial features rated as masculine (square-jawed, rugged, that sort of thing) are the result of high testosterone levels. There is a good evolutionary reason why such features should be attractive, and it is that such faces also indicate a strong immune system.

One of the unfortunate side-effects of being male is a higher death rate, at any given age, than if you are female. Part of the cause of this is that testosterone suppresses the immune system, leaving high-testosterone individuals particularly vulnerable to infection. So a man who has made it to sexual maturity despite his high testosterone levels probably has a particularly good immune system, which he can pass on to his children.

That is the good news. The bad news is that a man with high testosterone is more likely to love you and leave you, so you might want to settle for Mr Nice-guy and his more effeminate features. Not surprisingly, then, the data are equivocal about which sort of face women prefer.

The problem that Dr DeBruine spotted is that this equivocation might be real—with different women pursuing, albeit unconsciously, different reproductive strategies—or it might be an artefact of the way the experiments have been conducted. These sorts of studies usually rely on photographs that have been manipulated to make the faces in them look either more masculine, or less so. Since different groups of researchers have hit on different methods of manipulation, it might be that the women involved in the experiments are reacting to something about the methods in a way that is different from their reaction to real faces.

Dr DeBruine and her colleagues identified three manipulation methods. One works by extrapolating the shape differences between average men's and women's faces and morphing a man's face accordingly. Another uses a range of faces, independently rated for masculinity, as its starting point, and also allows a role for colour as well as shape. The third relies on tracking the way men's faces actually do change through puberty. Using the same basic faces, the team generated six versions of each—a masculinised and a feminised pair for each of the three manipulation methods.

What they found was that the manipulation method made no difference. Women really do vary in the degree of masculinity they prefer. Good news for wimps, then. Moreover, Dr DeBruine found that women's preferences as measured by the test agreed closely with the perceived degree of masculinity of the real-life partners of those who were in stable relationships. Every pot, as the proverb has it, finds its cover.

But it is not all bad news for the hyper-masculine. Past research has suggested that, regardless of their average preferences, women are most attracted to hyper-masculine features when they are most likely to conceive, and that the effect is particularly exaggerated in women who are in stable relationships. Evolution has thus arranged things so that if a woman does cuckold her man, she is likely to gain the maximum advantage in terms of children with good immune systems, and sons who will have similarly rakish good looks and behaviour. Just don't tell your husband that.