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Darwin does wonder about the quality of his data – and rightly so. But in general he is quite convinced that different people perceive beauty in different ways. His vision is an appealing one. Per molto variare la natur è bella – nature’s beauty is its variety; it could be Darwin’s slogan and it could be Benetton’s (though it was Elizabeth I’s). Indeed, when we consider the whirlwind of fashion it is impossible to doubt that the love of beauty is frequently the particular love of rare and meaningless things. Among scientists who study beauty, however – and the study of beauty is itself increasingly fashionable – Darwin’s views are seen as rather quaint. These days, most research on the subject begins with the notions that the standard of beauty is universal, that the presence of beauty is rather common, and that far from being meaningless, it has a great deal to say.

The universality of beauty’s standard is as self-evident as its particularity. The apparent contradiction is resolved if we simply recognise that there are some things about which tastes differ, and some about which they do not. Tastes in hairiness (cranial, facial, bodily), pigmentation (eye, hair and skin colour) and perhaps even body shape (hip–waist ratio) all seem to differ quite a lot from person to person, place to place, time to time. But the taste for relative youth – at least when men judge women – does not. Nor, it seems, does the taste for certain kinds of faces. Average faces seem to be universally more attractive than most, but not all, variant faces. Symmetry is preferred over asymmetry. These are some of the results of a large literature devoted to finding out who finds what beautiful when. Much of it demonstrates the obvious. After all, were a Papua New Guinea tribesman brought to London’s National Gallery and offered the choice of Botticelli’s Venus (she of Mars and Venus) and Massys’s Grotesque Old Woman as a mate, he might well be unimpressed by either, but we can be sure which one he’d choose.

Beauty’s meaning is more controversial. Here I wish to pursue just one idea: that it has something to do with physiological condition; that it is, indeed, a certificate of health. In its simplest form the truth of this idea is also quite self-evident. Clear skin, bright eyes and white teeth are manifestly signs of beauty and health. It is no accident that Brazilian men, glimpsing a beautiful carioca, sigh ‘Que saúde’ – what health. Whether particular facial proportions and symmetry signify health is, however, less obvious. Studies using computer-generated faces show that we perceive beautiful faces as being healthy ones. But searches for a correlation between the beauty and health of real people have found only weak and inconsistent effects.

Perhaps this is because beauty is no longer what it was. For all of human history, poor health has mostly been about nutrition and pathogens – a lack of the first and an excess of the second. Beauty was an indicator of the salubriousness of the environment or else the ability to resist its vicissitudes. To the degree that this is true, then the variance in beauty must be declining in the most developed nations at least, even as its mean increases. Goitres and cretinism may still afflict large parts of the world, but they no longer afflict the Swiss. The scars of smallpox have disappeared everywhere. Even in England most people now keep their teeth until they die. One wonders whether the diseases – filariasis, malaria, sleeping sickness, not to mention nutritional deprivation in its many forms – that afflict so many of the world’s children can be read in the symmetry and proportions of their faces if, as adults, they should have survived them. There is no doubt that prosperity exacts a cost to beauty in the form of obesity, dental cavities and stress. But if the balance of its effects is favourable, and it must be, then any classroom of American or European undergraduates contains an abundance of beauty that has never existed in human history before.

That may seem implausible, but only because we have little grasp of beauty’s advance. Beauty is like wealth. It increases over time, yet its distribution remains unequal. However much of it we have, it always seems that someone else has more. In part this is because beauty, as the consequence of health, is also the consequence of wealth. But suppose there existed a society so wealthy and egalitarian that, as far as pathogens and nutrition are concerned, all were equally healthy. A society of the sort approached by the Netherlands (but from which Great Britain and the United States remain woefully distant), in which the socioeconomic background of a child cannot be judged from his or her physical appearance alone. Would all be equally beautiful in such a society? Would beauty’s difference have disappeared? I doubt it. However beautiful the average Dutchman may believe himself to be, some of his compatriots will be more beautiful yet. I suspect that there is a residual variance in beauty that even the most controlled upbringing cannot eradicate. A residuum that lies in our genes.

The effects of poor childhood nutrition and exposure to pathogens upon the face may be uncertain, but the effects of mutations are not. When clinical geneticists attempt to classify the symptoms that their patients present, it is to the face that they first look. They are expert in recognising the subtleties that are often the only outward sign of deeper disturbances in the genetic order: shallow philtrum, low-set ears, upturned nose, narrow or wide-set eyes. Many, perhaps most, of the disorders that I have discussed in this book – from achondroplasia to pycnodysostosis – can be read in the face.

It seems that our faces are very vulnerable to mutation. Or perhaps we are just very good at reading mutation’s effects in them. Either way, it seems likely that mutation’s effects are written on all of our faces – not simply the faces of people with identified clinical disorders. I began this book by observing that every newly conceived embryo has, at an educated guess, an average of three hundred mutations that affect its health for the worse. It may seem impossible that we could, as a species, be so poorly. But a certain number of mutations are eliminated by selection in the womb. A woman who knows that she is pregnant has a 15 per cent chance of miscarriage; many more embryos must be lost to women who are unaware that they have conceived. More than 70 per cent of spontaneously aborted foetuses bear severe chromosomal abnormalities, and it is likely that many also bear mutations in particular genes. It is now widely supposed that miscarriage is an evolved device that enables mothers to screen for, and rid themselves of, genetically impaired progeny.

Mutation is a game of chance, one we must all play, and at which we all lose. But some of us lose more heavily than others. Some calculations hint at the distribution of our losses. If we suppose that, of the three hundred mutations that burden the average newly conceived embryo, five are lost from the population each generation by death (miscarriage, infant and childhood mortality), then the average adult carries 295 deleterious mutations. The least burdened 1 per cent of the population will have about 250 mutations, and the most burdened 342. Somewhere in the world there is a person who has the fewest mutations of all, about 191 of them.

These calculations confirm the intuition that no one leaves the genetic casino unscathed. But they are just educated guesses. They also take no account of the relative cost of each mutation. They are the equivalent of estimating gambling losses by counting the number of chips surrendered to the house without noting their value. It seems likely that the cost of most mutations is quite small. They give us minor ailments such as bad backs and weak eyes. I suspect that they also give us misaligned teeth, graceless noses and asymmetrical ears. If this is so, then the true meaning of beauty is the relative absence of genetic error.