The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (37 page)

From a sexual selection viewpoint, clitorises should respond only to men who demonstrate high fitness, including the physical fitness necessary for long, energetic sex, and the mental fitness necessary to understand what women want and how to deliver it. The choosy clitoris should produce orgasm only when the woman feels genuinely attracted to a man's body, mind, and personality, and when the man proves his attentiveness and fitness through the right stimulation.

Not surprisingly, female scientists have held the clitoris in higher regard than have male scientists. Helen Fisher, Meredith Small, and Sarah Blaffer Hrdy have viewed the clitoral orgasm as a legitimate adaptation in its own right, with major implications for female sexual behavior and sexual evolution. Lynn Margulis has pointed out that female orgasm leads to female choice, and female choice is how females influence the evolutionary trajectory of their species. Natalie Angier's recent book
Woman: An Intimate Geography
stressed the clitoral orgasm's role in sexual choice: "She is likely to have sex with men she finds attractive, men with whom she feels comfortable for any number of reasons, and thus to

further her personal, political, and genetic designs." I agree that the clitoris is an adaptation for sexual choice, and want to go one step further in considering its design within a sexual selection framework.

The sex difference between penis and clitoris can be viewed as a physical manifestation of Fisher's runaway process: a highly developed male trait (the penis) designed to stimulate, and a highly discerning female preference (the clitoral orgasm) designed to respond selectively to skillful stimulation. If this runaway model is right, then there was a sort of stimulatory arms race between the human penis and the human clitoris. The penis evolved to deliver more and more stimulation, while the clitoris evolved to demand more and more.

This tension explains why women and men are not well adapted to giving each other easy, simultaneous, repeated orgasms. If the function of orgasm were simply to reinforce monogamous pair-bonds, why should evolution make female orgasm so difficult and male orgasm so easy during vaginal intercourse? If female orgasm is a side-effect of male orgasm, why does it just happen to work when an attractive man provides a lot of foreplay and deep, slow copulatory thrusting, but not so well when sex is hurried or the partner is undesirable? Surely, sexual selection theory offers insight into this ancient human mystery. Female orgasm seems poorly designed as a pair-bonding mechanism, but it is perfectly designed as a discriminatory system that separates the men from the boys.

Yet the image of an evolutionary arms race between penis and clitoris is not quite accurate. The female mechanism for assessing penis size is not the clitoris itself, but the ring of nerves around the entrance to the vagina, which sense circumference. The clitoris does something more sophisticated, assessing the male's ability to move in pleasurable, rhythmic ways during copulation. Also, clitoral stimulation usually leads to orgasm only when the female mind is feeling erotic about the man and the situation. Human female orgasm depends on an interaction between the clitoris, the hypothalamus (the brain's emotional center), and the cerebral

cortex (the brain's cognitive center). The clitoris is only the tip of

the psychological iceberg in female choice. Having a mate with a
large penis is not enough. To be fair, the penis is not just an

insensate stimulator either. It is also a mechanism for male mate choice. If it is happy, its owner may be more likely to stay in a longterm relationship with a woman.

Tragically, while scientists in developed countries spent decades debating whether clitorises are legitimate adaptations, over a

hundred million clitorises were cut out of African girls by village

women precisely so that the girls would not be tempted to exercise
their powers of sexual choice. Currently, another two million girls

a year are genitally mutilated in countries such as Egypt, Sudan, Somalia, and Ethiopia. To my mind, sexual selection theory offers a powerful scientific rebuttal to the argument that we should accept female genital mutilation in such countries as part of "traditional tribal practice."

Just as the penis can be seen as a metaphor for the mind's sexually selected entertainment abilities, the clitoris can be seen as

a metaphor for the mind's judgment and discrimination abilities.

When we see a human perceptual or cognitive ability that looks
curiously sensitive to stimulation yet resistant to satisfaction, we

should not assume that it is a poorly designed information pro-

cessing system. It may be part of a system for sexual or social
discrimination. Consider humor. Some theories of humor have

proposed that laughter evolved to promote group bonding, discharge nervous tension, or keep us healthy. The more laughter the better. Such theories predict that we should laugh at any joke, however stupid, however many times we have heard it before, yet we do not. A good sense of humor means a discriminating sense of humor, not a hyena-like shriek at every repetitive pratfall. Such discrimination is easy to understand if our sense of humor evolved in the service of sexual choice, to assess the joke-telling ability of others.

Breasts

produce milk for feeding offspring. Any discussion about the evolution of breasts has to take this mammalian heritage as the starting point. Milk-substitute manufacturers have worked very hard for almost a century to convince women that they are not mammals and have no business breast-feeding. Even many science journalists support this view, as when some recent research was reported as showing that "breast-feeding raises IQ by five points," rather than "bottle-feeding reduces IQ by five points"—as if bottle-feeding was the biological norm. The popularity of bottle-feeding and breast implants should not mislead us into viewing breasts as nothing more than sexual ornaments.

During human evolution, female breasts would have been producing milk about half of the time between puberty and menopause. Babies probably nursed for at least a year or two, as they do in hunter-gatherer societies today. Without contraception, after a mother stopped nursing one baby she would typically have conceived the next baby within a few months. Assuming that the average female hominid produced at least 20 fluid ounces of milk per day when breast-feeding, and she spent a total of ten years breast-feeding in her life, the average hominid breast would have delivered over 35,000 fluid ounces (nearly 300 U.S. gallons) before menopause.

This high level of milk production does not itself explain why female humans breasts are so much larger than those of other apes. Most primate females are quite flat-chested, even when producing milk. Milk output depends on the amount of active glandular tissue in the breast, not the volume of fat. Human breasts have an unusually high ratio of fat to glandular tissue. They do not seem to be optimized for milk production. Most experts on breast-feeding claim there is no correlation between breast size before pregnancy and milk production ability after birth (though I know of no good data on this point). Milk output seems limited more by a woman's overall nutritional state than by her pre-pregnancy breast size. So, we have to distinguish between mammary glands, which evolved for milk production, and enlarged human breasts, which must have evolved for something

else. It seems likely that sexual selection played a role. But how?

Perhaps breasts evolved as cues of sexual maturity. Human breasts enlarge at puberty long before they are required for breast-feeding the first baby. Just as bipedal walking may have allowed female choice to focus more on the penis, bipedalism may have allowed male choice to focus on female breasts as a maturity cue. However, maturity cues do not have to be so dramatic. Males have evolutionary incentives to distinguish mature women from infertile girls, women have evolutionary incentives to advertise their fertility, and girls have evolutionary incentives to advertise their infertility. Given these shared interests, signals of sexual maturity could be very inconspicuous. Males of most other species have no trouble distinguishing mature from immature females using relatively subtle cues.

It seems likely that male choice shaped breasts not to distinguish girls from women, but to distinguish young women from older women. Here, the informative thing about breasts is the way they droop with the effects of age and gravity. There is a relatively narrow age window in which large breasts can appear pert before repeated cycles of pregnancy and breast-feeding cause them to sag. There were no bras or breast-lift operations in the Pleistocene. As we saw in the previous chapter, hominid males probably favored younger women for their higher fertility. Any indicator of youth, such as large, pert breasts, would tend to be favored by males. A male preference for size and pertness would spread at the expense of male preferences for droopiness and flatness, because the latter preferences would generally lead men to choose older, less fertile partners.

This argument sounds fine from the male point of view, but it takes a bit of thought to see why females should evolve youth indicators. The most informative cues of youth are also the most informative cues of age. Youth indicators might make women more attractive when they are truly young, but might make them less attractive when they are older. A mutation that caused an enlargement in breast size might benefit its carriers when they are in their teens and twenties, but impose high costs when they are in

their thirties and forties. The question is whether the early benefits would outweigh the later costs. The answer is probably yes, because it is almost always better to have babies earlier than later in life. Females tend to be more fertile in youth, produce fewer birth defects, are in better shape to care for offspring, and are more likely to have living sisters and mothers to help with childcare. Also, fast breeders produce more generations per century, so can increase their population numbers faster than slow breeders. For these reasons an attractiveness benefit in youth can often outweigh an unattractiveness cost in older age. This is why it can be in the interest of females to evolve youth indicators such as large breasts that tend to droop, fine skin that tends to wrinkle, and buttocks that tend to develop stretch marks. This is one of the most counter-intuitive applications of Zahavi's handicap principle.

Breasts also make good fitness indicators because they come in symmetric pairs. I mentioned earlier that many bodily ornaments in many species advertise an aspect of fitness called developmental stability When body traits grow in pairs, perfectly symmetric development of the pair indicates high fitness. The paired traits tend to grow large to make their symmetry more obvious during mate choice. Evolutionary psychologists John Manning and Randy Thornhill have shown that women with more symmetric breasts tend to be more fertile. It is possible that bipedalism made breasts a useful potential cue of developmental stability for male mate choice. Once men started paying attention to the symmetry of breast development, high-fitness women could better display the symmetry by evolving large breasts. The larger the breasts, the easier it is to notice asymmetries. Perhaps single mastectomies are so distressing to women because breast symmetry has been such an important fitness cue during human evolution. Large human breasts may have evolved to advertise fitness through their symmetry, not just youth through their pertness.

Finally, breasts are pretty good indicators of fat reserves. In the Pleistocene, starving was more of a problem than overeating. It was harder to have good fat reserves than to be extremely thin,

because women had to use their own energy and intelligence to gather food from their environment. It would be possible to spread one's fat evenly over the whole body surface, like a porpoise, but that would make it hard for men to compare females, and it would give females too much insulation under the scorching African sun. Females who concentrated their fat-displays in breast and buttocks could attract male interest without overheating. Also, by not depositing too much fat on the abdomen (as males tend to), females could avoid appearing pregnant already—a sure sign of not being fertile at the moment, which might inhibit male sexual attention. Breasts appear to have evolved as highly condition-dependent indicators of a woman's nutritional state. Most women who have tried dieting know that breast size is the first thing to shrink when food intake is restricted.

The role of breasts as fitness indicators may help to explain why there is so much variation in breast size among women. If large breasts were critical for breast-feeding, which is one of the single most important stages in mammalian reproduction, all women would have large breasts. But as we have seen, fitness indicators do not tend to converge on a single size in a population. They maintain their variation indefinitely, due to the effects of genetic mutation and variation in condition. It has sometimes been argued that men's preferences for larger-than-average breasts must be an artifact of modern culture, because, if it were ancient, all women would have already have evolved large breasts. This argument is wrong if breasts evolved as fitness indicators. Bra manufacturers offer a range from A-cups to D-cups because evolution amplifies the variation in each fitness indicator rather than using it up.

However, even more important in explaining such variation is the fact that each sex assesses the other using a wide range of fitness indicators. This leads to surprising and subtle effects. Imagine that each indicator advertises a different aspect of physical or mental fitness. Because each indicator is costly (so it works according to the handicap principle), there are trade-offs between indicators. This allows scope for individuals to differ in