The Red Queen: Sex and the Evolution of Human Nature

by Matt Ridley

True, this will not avert marriage between cousins, but then there is nothing much wrong with marriage between cousins: The chance of a recessive deleterious gene emerging from such a match is small, and the advantages of genetic alliance to preserve complexes of genes that are adapted to work with one another probably outweigh it. (Quail prefer to mate with first cousins rather than with strangers.) Westermarck did not know that, of course, but it strengthens his argument, for it suggests that the only incestuous relations a human being should avoid are the ones between brother and sister, and parent and child.

Very close childhood friends would also generally not be found to marry. Here the best evidence comes from two sources: Israeli kibbutzim and an old Chinese marriage custom. In kibbutzim, children are reared in crèches with unrelated companions. Lifelong friendships are formed, but marriages between fellow kibbutz children are very rare. In Taiwan some families practice “shimpua marriage” in which an infant daughter is brought up by the family of the man she will marry. She is therefore effectively married to her stepbrother. Such marriages are often infertile, largely because the two partners find each other sexually unattractive. Conversely, two siblings reared apart are surprisingly likely to fall in love with each other if they meet at the right age.

But Westermarck’s theory would also predict that if incest does occur, it will prove to be between parent and child, and specifically between father and daughter, because a father is past the age at which familiarity breeds aversion and because men usually initiate sex. That, of course, is the most common form of incest.

The incest story neatly demonstrates the interdependence of nature and nurture. The incest avoidance mechanism is socially induced: You become sexually averse to your siblings during your childhood. In that sense there is nothing genetic about it. And yet it is genetic, for it is not taught: It just develops within the brain. The instinct not to mate with childhood companions is nature, but the features by which you recognize them are nurture.

Few people change their accents after the age of about twenty-five, even if they move from, say, the United States to Britain. But if they move at ten or fifteen, they quickly adopt a British accent. They are just like white-crowned sparrows, which sing with the dialect of the place where they lived at two months old.

We become sexually indifferent to those with whom we were reared during a critical period. Nobody is certain exactly what constitutes the critical period, but it is a plausible guess that it lasts from, say, eight to fourteen, the years before puberty.

Today, as in the Pleistocene period, that is a sure way to choose the least fertile woman: A woman can be rendered infertile by a body fat content only 10-15 percent below normal. Indeed, one theory is that the widespread obsession of young women with their weight is an evolved strategy to avoid getting pregnant too early or before a man has committed himself to raising a family.

There is ample evidence from sculpture and painting that Victorian beauties were not especially thin, and from sculpture and painting as far back as the Renaissance that beautiful women were plump women. There are exceptions. Nefertiti’s neck was that of a thin, elegant woman. Botticelli’s Venus was not exactly overweight. And for a time, Victorians worshiped at the shrine of wasp waists, so much so that some women allegedly removed a pair of ribs to make their waists slimmer.

As Robert Smuts of the University of Michigan has argued, thinness was once all too common and was a sign of relative poverty. Nowadays, poverty-induced thinness is confined to the Third World. But in the industrialized nations, wealthy women are able to afford a diet low in fat and spend their money on dieting and exercise. Thinness has become what fatness was: a sign of status.

A young man growing up today is bombarded with correlations between thinness and wealth, from the fashion industry in particular. His unconscious mind begins to make the connection during his critical period, and when he is forming his idealized mental preference for a woman, he accordingly makes her slim.

Several studies have come to the unambiguous conclusion that beautiful women and rich men end up together far more than vice versa. In one study the physical attractiveness of a woman was a far better predictor of the occupational status of the man she married than her own socioeconomic status, intelligence, or education—a rather surprising fact when you consider how often people marry within their profession, class, and education brackets.

He argues that, within reason, a man will find almost any weight of a woman attractive as long as her waist is much thinner than her hips.

Most apes give birth to babies whose brain is half-grown; human babies’ brains are one-third grown at birth, and they spend far less time in the womb than is normal for a mammal, given the longevity of man. The reason is obvious: Were the hole in the pelvis through which we are born (the birth canal) commensurately larger, our mothers would be unable to walk at all. The width of human hips reached a certain point and could go no further; as brains continued to grow bigger, earlier birth was the only option left to the species.

Imagine the evolutionary pressure of this process on female hip size. It was always wise for a man to choose the biggest-hipped woman he could find, generation after generation, for millions of years. At a certain point hips could get no bigger but men still had the preference, so women with slender waists who appeared to have larger hips by contrast were preferred instead.

A man cannot tell the age of a woman directly. He must infer it from her physical appearance, her behavior, and her reputation. It is intriguing to note that many of the most noticed features of female beauty decay rapidly with age: unblemished skin, full lips, clear eyes, upright breasts, narrow waists, slender legs, even blond hair, which, without chemical intervention, rarely lasts beyond the twenties except among the most Viking of people.

The evidence is beginning to accumulate that humanity is a highly sexually selected species and that this explains the great variations between races in hairiness, nose length, hair length, hair curliness, beards, eye color—variations that plainly have little to do with climate or any other physical factor. In the common pheasant, every one of forty-six isolated wild populations in central Asia has a different combination of male plumage ornaments: white collars, green heads, blue rumps, orange breasts. Likewise, in mankind, sexual selection is at work.

If a man is to devote his life to a wife, he must know that she has a potentially long reproductive life ahead of her. If he were to form occasional short-lived pair bonds throughout his life, it would not matter how young his mates were. We are, in other words, descended from men who chose young women as mates and so left more sons and daughters in the world than other men.

In 1883, Francis Galton discovered that merging the photographs of several women’s faces produced a composite that is usually judged to be better looking than any of the individual faces that went into making it.25 The experiment has been repeated recently with computer-merged photographs of female undergraduates: The more faces that go into the image, the more beautiful the woman appears.26 Indeed, the faces of models are eminently forgettable. Despite seeing them on the covers of magazines every day, we learn to recognize few individuals. The faces of politicians, not known for their beauty, are much more memorable. Faces that are “full of character” are almost by definition nonaverage faces. The more average and unblemished the face, the more beautiful, but the less it tells you about its owner’s character.

Given the importance of facial beauty, a man who chooses an ugly-faced mate will probably have daughters that marry late or marry second-rate husbands. Throughout human history men have fulfilled their ambitions through their daughters’ looks. In societies with few other opportunities for social mobility, a great beauty could always marry above her station.

What draws women to certain men? Male handsomeness is affected by the same trinity as female beauty—face, youth, and figure. But in study after study, women consistently agree that these factors matter less than personality and status. Men consistently place physical features above personality and status when considering women; women do not when considering men.

The single exception is height. Tall men are universally considered more attractive by women than short men. In the world of dating agencies, the principle that a man must be taller than his date is so universal that it has been called “the cardinal principle of date selection.” Out of 720 applications by couples for bank accounts, only one was from a couple in which the woman was taller than the man, and yet couples chosen at random from the population would show scores of such cases.

Bruce Ellis has summarized the evidence that personality is critical in men. In a monogamous society a woman often chooses a mate long before he has had a chance to become a “chief,” and she must look for clues to his future potential rather than rely only on his past achievements. Poise, self assurance, optimism, efficiency, perseverance, courage, decisiveness, intelligence, ambition—these are the things that cause men to rise to the top of their professions. And not coincidentally, these are the things women find attractive. They are clues to future status.