BEAUTY, the saying has it, is only skin deep. Not true. Skin is important (the cosmetics industry proves that). But so is what lies under it. In particular, the shape of people’s faces, determined by their bone structure, contributes enormously to how beautiful they are. And, since the ultimate point of beauty is to signal who is a good prospect as a mate, what makes a face beautiful is not only an aesthetic matter but also a biological one. How those bone structures arise, and how they communicate desirable traits, are big evolutionary questions.

Until now, experiments to try to determine the biological basis of beauty have been of the please-look-at-these-photographs-and-answer-some-questions variety. Some useful and not necessarily obvious results have emerged, such as that one determinant of beauty is facial symmetry.

But what would really help is a breeding experiment which allowed the shapes of faces to be followed across the generations to see how those shapes relate to variations in things that might be desirable in a mate. These might include fertility, fecundity, social status, present health, and likely resistance to future infection and infestation. Correlations between many of these phenomena and attributes of the body-beautiful have, indeed, been established. But in a pair-forming, highly social species such as Homo sapiens, you also have to live with your co-child-raiser or, at least, collaborate with him or her. So other things may be important in a mate, too, such as an even temper and a friendly outlook.

It would be impossible to do such a breeding experiment on people, of course. But as Irene Elia, a biological anthropologist at Cambridge, realised, it has in fact been done, for the past five decades, on a different species of animal. Dr Elia has published her analysis of this experiment in the Quarterly Review of Biology. The animals in question are foxes.

Foxy ladies, vulpine gents

The story starts in 1959, in Novosibirsk, Russia. That was when Dmitry Belyaev, a geneticist, began an experiment which continues to this day. He tried to breed silver foxes (a melanic colour variant, beloved of furriers, of the familiar red fox) to make them tamer and thus easier for farmers to handle. He found he could, but the process also had other effects: the animals’ coats developed patches of colour; their ears became floppy; their skulls became rounded and foreshortened; their faces flattened; their noses got stubbier; and their jaws shortened, thus crowding their teeth.

All told, then, these animals became, to wild foxes, the equivalent of what dogs are to wild wolves. And this was solely the result of selection for what Belyaev called “friendly” behaviour—neither fearful nor aggressive, but calm and eager to interact with people.

The link appears to be hormonal. Hormones such as estradiol and neurotransmitters such as serotonin, which regulate behaviour, also regulate some aspects of development. Change one and you will change the other. So in a species where friendliness is favoured because that species is social and the group members have to get on with each other—a species like Homo sapiens, for example—a “friendly” face is a feature that might actively be sought, both in mates and in children, because it is a marker of desirable social attitudes. And there is abundant evidence, reviewed by Dr Elia, both that it is indeed actively sought by Homo sapiens, and that it is such a reliable marker.

What men look for in the faces of women, and vice versa, is so well known that research might seem superfluous. Suffice to say, then, that features like those seen in Belyaev’s foxes (flat faces, small noses, reduced jaws and a large ratio between the height of the cranium and the height of the face) are on the list. People with large craniofacial ratios are, literally, highbrow.

More intriguingly, the presence or absence of such features skews parents’ attitudes to their offspring. At least 15 studies have shown that mothers treat attractive children more favourably than unattractive ones, even though they say they don’t and may actually believe that. At least one of these studies showed this bias is true from birth.

Some of the details are extraordinary. One researcher, who spent a decade observing how mothers look after young children in supermarkets, found that only 1% of children judged unattractive by independent assessors were safely secured in the seats of grocery carts. In the case of the most attractive the figure was 13%. Another researcher studied police photographs of children who had been abused and found such children had lower craniofacial ratios than those who had not been.

In a state of nature, this sort of behaviour would surely translate into selective death and thus the spread of the facial features humans are pleased to describe as “beautiful”. If such features do indicate a propensity to friendly, sociable behaviour, as they do in foxes, then such behaviours will spread too.

Crucially for Dr Elia’s hypothesis, they do indeed indicate such a propensity. Even as children, according to 33 separate studies, the attractive are better adjusted and more popular than the ugly (they also have higher intelligence, which assists social skills). And of course, they have less difficulty finding a mate—and as a result have more children themselves. One study found that the most beautiful women in it had up to 16% more offspring than their less-favoured sisters. Conversely, the least attractive men had 13% fewer than their more handsome confrères.

The beholder’s eye

An appreciation of what is “beautiful”, moreover, seems innate—as Dr Elia’s hypothesis requires it should be. Babies a few days old prefer pictures of the faces of people whom their elders would define as beautiful to those they would not, regardless of the sex and race of either the baby or the person in the photo.

People also seem to be more beautiful now than they were in the past—precisely as would be expected if beauty is still evolving. This has been shown by assessing the beauty of reconstructions of the faces of early humans. (Such reconstructions, sometimes used in murder cases where only skeletal remains of the victim are available, produce reliable depictions of recently dead people, so the assumption is that ancients really did look like the reconstructions made of them.)

None of this absolutely proves Dr Elia’s hypothesis. But it looks plausible. If she is right, facial beauty ceases to be an arbitrary characteristic and instead becomes a reliable marker of underlying desirable behaviour. It is selected for both in the ways beautiful children are brought up, and in the number of children the beautiful have. Face it.