The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (33 page)

Age and Fertility

The most important quality that indicators advertise other than heritable fitness is age. Obviously, age is not directly heritable. A 40 -year-old woman will give birth to a nine-month-old, just as a 20-year-old woman will. However, age has a dramatic effect on fertility, especially in women. Individuals before puberty are infertile. Female adolescents are significantly less fertile than 20-

year-olds. Female fertility declines gradually during the thirties, and declines steeply after age 40. Women after menopause are infertile. This female fertility profile is a basic fact of life to which male mate choice systems have adapted. Youth is an important cue of fertility.

There may have been male hominids who preferred to start exciting relationships with wise, fulfilled, 60-year-old females. But if they did so exclusively, they would have left no offspring to inherit that preference. Any sexual choice mechanism that preferred infertile individuals to fertile individuals would have died out in one generation. Since male sperm production ability declines more slowly with age, female mate preferences need not have paid so much attention to a man's age as a cue of his reproductive ability. This reasoning, as developed by Don Symons, David Buss, and other evolutionary psychologists, explains the universal, cross-cultural pattern that men care more about a partner's age than women do, men generally preferring partners younger than themselves, and women generally preferring partners older than themselves.

However, male hominids may not have been quite so youth-obsessed as men from agricultural, pastoral, and modern civilizations. In most cultures with recorded history, men were under social, legal, economic, and religious pressures to stay monogamously married for life. The younger their bride, the more offspring they could produce. This put a huge premium on youth, and men competed to claim young women before another man could.

A woman's youth may not have been quite so crucial in the Pleistocene, as long as the woman was still reasonably fertile. If our hominid ancestors had several medium-term relationships in sequence, males need not have been so picky about female age. If the relationship was likely to end after five years—as anthropologist Helen Fisher has argued that they usually did in prehistory—it would have mattered little whether she was 10 years or 30 years away from menopause.

During her reproductive years, a woman's age does have a

negative correlation with her fertility. But under challenging Pleistocene conditions, age would have had a positive correlation with heritable fitness because low-fitness individuals would have died younger. Any woman who managed to reach her mid-thirties and raise several children successfully while staying physically and psychologically attractive, might have made a better genetic bet for a choosy male than an untested teenager of unproven fertility. Other male primates tend to shun adolescent females without offspring, and prefer older, high-ranking females with offspring who have already demonstrated their fertility, survival ability, social intelligence, and mothering skills.

There is strong evidence from evolutionary psychology that men in modern societies generally prefer the physical appearance of women around 20 years old to those who are older (or younger). But I have argued that this preference may have been amplified somewhat by the economic and religious pressures for monogamy since civilization arose, which makes finding a young bride crucial to a man's reproductive success.

More importantly, there has been much less research on the age at which women's minds are most attractive. Perhaps mature men tend to find young women beautiful but boring, and older women slightly less physically attractive but much more interesting. If so, we should not view the preference for youthful appearance as any less of a legitimate adaptation than the preference for a worldly mind. Data gathered by Doug Kenrick shows that older men generally prefer women closer to their own age—in their mid-thirties rather than their early twenties, for example—as longterm sexual partners. Presumably this is because women in their mid-thirties are typically more intriguing, multifaceted people who display the mental aspects of their fitness in richer ways that can be more reliably assessed. Evolutionary psychology has rightfully emphasized the strong male human interest in young female bodies, but I think its scope should be broadened to include the romantic interest aroused in both sexes by mature, worldly minds.

In any case, chronological age, like heritable fitness, could not

be perceived directly during human evolution. To distinguish children from adults, our ancestors had to rely on cues of sexual maturity such as male musculature, beard growth, and voice pitch, and female breast and hip development. To distinguish young adults of peak fertility from other adults of declining fertility, they had to rely on age cues such as wrinkles, gray hair, sagging skin, slow gait, and memory loss.

Like fitness indicators, age indicators leave some room for deception. This may have some relation to our apparent "neoteny," which means that we have, it has been argued, retained some of the physical and mental traits of juvenile apes into our adulthood. Our faces look more like the faces of very young chimpanzees than they do like those of adult chimpanzees. Our playful creativity resembles the behavior of young primates more than it does the stern, lazy brutality of adult apes. Stephen Jay Gould has argued that our neotenization was a key trend in human evolution, and he sees our behavioral flexibility as a side-effect of our general neoteny.

But neoteny can be viewed very differently. Our neotenous features may have evolved through sexual choice as somewhat deceptive cues of youth. If male hominids preferred younger, more fertile females to older, less fertile females, then there would have been sexual selection pressures on females to appear physically and behaviorally younger than they really were. They could do this by evolving younger-looking faces, and by being more playful, creative, spontaneous, and uninhibited throughout their adult life. The result would be neotenized female hominids. The same argument could apply to males, insofar as female choice favored signs of youthful energy. (It is not clear why our lineage evolved these neotenous youth-cues while other primates did not—one could invoke sexual selection's unpredictability, though that is not a very satisfying explanation.) In my view, Gould's neoteny theory identified a set of somewhat deceptive youthfulness indicators that must have evolved through some form of sexual or social selection. It is not a competing theory of human evolution, but a description of some physical and psychological trends that still require an evolutionary explanation.

Apparent preferences for youth are not as simple as they seem. It is often hard to distinguish indicators of youth from indicators of fitness. This is because fitness indicators usually work by being very dependent on condition, and condition is highest during the flower of youth. All things being equal, any mate choice mechanism that evolved to favor a condition-dependent indicator will tend to favor youth over age simply because youths will display the indicator in a healthier condition. However, the fact that women often prefer older men suggests that mate choice mechanisms can easily evolve to compensate for this youth-bias whenever it proves maladaptive.

Fitness Indicators for People Other than Mates

Sexual selection was not the only kind of social selection during human evolution. For humans, as for most primates, all kinds of social relationships affect survival and reproduction. In forming and maintaining many of these relationships there are good reasons to advertise one's fitness, just as one does to potential sexual partners. Friends of higher fitness may survive longer, offer more competencies, and give better advice. Allies of higher fitness may help one to win fights and wars. Trading partners of higher fitness may live longer, travel longer distances to acquire more valuable commodities, and have the social intelligence to keep their promises. None of these social relationships entails any merging of genes, so they are not subject to positive-feedback processes as powerful as runaway sexual selection. But they still offer plenty of scope for all kinds of socially selected indicators to evolve.

We can often use the same fitness indicators in non-sexual relationships as we do in sexual relationships. If vigorous dancing all night displays our physical fitness to potential mates, it equally displays our fitness to potential friends and allies. Whenever a fitness indicator evolved in our ancestors through sexual selection, it was probably generalized to other social relationships rather quickly. Conversely, any indicator that evolved in the context of

friendships or tribal alliances could easily have been modified for courtship.

The overlapping use of fitness indicators in sexual and nonsexual relationships is why making friends so often feels like a variant of sexual courtship. There is the same desire to present oneself to best advantage, emphasizing skills, downplaying weaknesses, revealing past adventures, investing extra energy in the interaction. This does not mean that friendships always have a sexual undercurrent, or that friendship is maintained through some kind of sexual sublimation. It simply means that the same principles of self-advertisement work in both kinds of relationship. If friendships gave important survival and social advantages during human evolution, and if our ancestors were choosy about their friends, then many of our fitness indicators may have evolved for friendship as well as for sexual relationships.

An especially important non-sexual relationship is that between parents and offspring. Children often compete to display their fitness to their parents, older siblings, and older relatives. They may shout "Hey dad, look at this!," and then try to do something that is challenging for a child of their age and abilities. At first glance it seems odd that they should bother. According to modern social norms, parents are supposed to love their children unconditionally, regardless of their fitness or abilities. But Pleistocene Africa did not always permit such unconditional support. Times were sometimes tough. Just as birds often have to choose which chick gets the worm and which starves, human parents may have had to choose how much support to invest in a particular child. Evolutionary psychologists Martin Daly and Margo Wilson have called this the problem of "discriminative parental solicitude." Parents must sometimes discriminate about which child deserves their solicitude. Older children are often favored because they have already survived the risky phase of infancy. But parents may also be sensitive to a child's fitness, which mean its prospects of successful survival and reproduction. Investment in a very low-fitness child means investment in an individual very unlikely to pass one's genes on to grandchildren. For better or worse,

evolution considers that an unwise investment, and favors a more discriminating attitude. In every culture, children with physical deformities and serious psychological disorders are at enormously greater risk of neglect, abuse, beating, and infanticide by parents.

Given parents who discriminate between children based on their apparent fitness, children have incentives to evolve fitness indicators. As when people initiate friendships, children can* use many of the same strategies that work in courtship, without there being any hidden sexual motive to the display This is where I believe Freud went wrong with his hypotheses about Oedipus and Electra complexes. He observed a set of fitness indicators that children directed at parents—energetic play, humorous storytelling, flirtatious conversation—and inferred a secret children's desire to have sex with their parents. That inference seems evolutionarily incredible. Presumably our hominid ancestors evolved a set of sexual choice mechanisms for judging the fitness of potential mates. Perhaps children found it convenient to play upon some of the same mechanisms to advertise their fitness to their parents, to solicit more attention and care. This does not mean that children want incest—it means that they want parental support.

Gay Hominids?

Homosexuality has not been mentioned so far in this book. My heterosexual emphasis comes not from homophobia, religious conviction, or moral conservatism. My subject is human evolution, and homosexual behavior is just not very important in evolution. Not a single ancestor of any living human was exclusively homosexual. Any hominid that was would not have produced any offspring, and would not have become anyone's ancestor. There may have been many gay and lesbian hominids, but if they were exclusively homosexual, they are not our ancestors, and we are not their descendants. In any case, it is unlikely that there were many exclusively homosexual hominids. Any genetic propensity towards exclusive homosexuality would have been eliminated in just one generation of selection. No

biologist has ever offered a credible theory explaining how exclusive homosexuality could evolve in a sexually reproducing species. Its existence in 1 or 2 percent of modern humans is a genuine evolutionary enigma that I cannot explain.

There is no such evolutionary problem with bisexuality, in which individuals enjoy sex with both sexes. Certainly bisexual behavior occurs in other species. Bonobos (previously known as "pygmy chimpanzees") engage in a lot of sexual activity with same-sex individuals, including kissing, genital rubbing, and genital licking. This does not impair their heterosexual reproduction in the slightest. Evolution does not respect our hunger for simplistic political categories of sexual behavior, in which every individual can be put on a continuum of "sexual orientation." Ordinary bonobos enjoy heterosexual behavior, and homosexual behavior, and they have lasted a million years as a species, about ten times longer than we have so far. There is nothing "unnatural" about homosexual behavior.