The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (15 page)

If genetic correlations between the sexes were not transient, we would never see dramatic sex differences in nature. Peahens would have the same tails as peacocks. Female nightingales would sing like males. The human clitoris would be as large as the human penis. Darwin's coattail theory of female brain evolution doesn't work except in the short term, because sex differences will eventually evolve if the sexes derive different benefits from ornamentation and sexual choice. Genetic correlations between the sexes can explain transient increases in female ornamentation and in male choosiness, but these increases are not evolutionarily stable. Fortunately, there is a second factor that is much more potent over the long term in keeping the sexes similar.

The Mental Capacities for Courtship Overlap
with the Capacities for Sexual Choice

The eye of the peahen has very little in common with the tail of
the peacock. They are at opposite ends of the body. They are
constructed of different materials. They grow under the influence

of different genes. During runaway, the genes underlying the sexually selected trait (the tail) may become correlated with the genes underlying the mechanism of sexual choice (the eye), but that is about the limit of their acquaintance.

The same is not true of the mental capacities used in human courtship, such as creative intelligence. There is much more overlap between those aspects of the brain used for producing sexually attractive behavior, and those aspects of the brain used for assessing and judging that behavior. Speaking and listening use many of the same language circuits. The production and appreciation of art probably rely on similar aesthetic capacities. It takes a sense of humor to recognize a sense of humor. Without intelligence, it is hard to appreciate another person's intelligence. The more psychologically refined a courtship display is, the more overlap there may be between the psychology required to produce the display and the psychology required to appreciate it.

This overlap suggests that runaway sexual selection for psychologically refined courtship may produce much smaller sex differences than runaway sexual selection for long bird tails. Consider the case of language. Suppose that human language evolved through a pure runaway process. Let's say males talked, and females listened, and females happened to favor articulate conversationalists over tedious mumblers. Male language abilities would then improve by sexual selection: their vocabularies might grow larger, their syntax more complex, their story plots more intricate, their ideas more imaginative. But for runaway to work, female choosiness would have to increase as well. How could that happen? Female language abilities would have to keep one step ahead of male abilities, to remain discerning. Females would have to be able to judge whether males used words correctly, so their vocabularies would keep pace. They would have to be able to notice grammatical errors, so their syntax abilities would keep pace. Most importantly, the females would have to understand what the males were saying to judge their meaning. Even if males exerted no sexual selection whatsoever on female language abilities, those abilities would have to evolve as part of the female mate choice mechanism.

To a psychologist like me, this is a much more promising sort of overlap than a mere genetic correlation between the sexes. There is a profound functional reason why males and females evolve in psychologically similar ways when courtship turns psychological. They use the same mental machinery to produce displays that they use to judge the displays produced by others.

There are two further reasons for the overlap between display-producers and display-judgers. To produce a really effective display, it helps to anticipate how the display will be judged. One might mentally rehearse a joke before telling it, to see if it will work, and find another joke if it won't. A painter could look at a picture while painting to see if it's beautiful. A musician could listen to the melody being played to see if it's tuneful. When trying to impress someone during courtship, we routinely do this sort of anticipatory filtering and correcting. Even if only males produced courtship displays, they would benefit by evolving psychological access to the same judgment mechanisms that females use.

Conversely, to be a really good judge of something, it helps to be able to do it oneself. For females to judge which male tells the best jokes, they may benefit by evolving joke-telling ability. We shall see later that mental anticipation is closely related to creativity. To be capable of judging someone's creativity, one must develop expectations about their behavior. Without expectations that can be violated, there can be no sexual selection for novelty and creativity. The mental machinery for generating expectations about someone else's stories, jokes, or music may overlap considerably with the mental machinery that is used in producing stories, jokes, and music.

So, even given a pure runaway process based on male courtship and female choice, male minds will tend to internalize the sexual

preferences of females in their own courtship equipment, in order

to produce better displays. And female minds will tend to internalize the display-production abilities of males in their own

sexual choice equipment, in order to be better judges of male displays. This should lead to many mental capacities being shared

by both sexes, even if males are more motivated to use their mental capacities to produce loud, public courtship displays. At present this argument is speculative, but it could be supported if neuroscience research found overlap between the brain areas used in producing and judging particular forms of courtship behavior, and if behavior genetic research were to show that the same genes underlie culture-production and culture-judgment abilities in both sexes.

Mutual Choice

Genetic and psychological overlaps between the sexes are fine as far as they go. They may explain some of the mental similarities between men and women, even if the pure runaway brain theory is right. Still, they raise two problems. First, they portray the female mind as riding along on the evolutionary coattails of the male mind, and female intelligence as an evolutionary side-effect of male intelligence. The runaway brain theory does put female brains in the evolutionary driver's seat, since they make the sexual choices that drive runaway sexual selection. But the males are portrayed as doing all the interesting things: the courtship displays, the storytelling, the music-making, the creative idea-work. In short, the runaway brain theory sounds sexist.

In the game of science though, sounding sexist is not a good reason to ban a theory. Science is the one zone of human thought where ideological preferences are not supposed to influence the assessment of ideas and evidence. Human evolution happened somehow. It may not have happened in a way that coincides with our ideological preferences. Usually, I have a very low tolerance when it comes to injecting ideology into discussions about human evolution. However, some objections that are expressed in ideological terms are actually empirical objections that have scientific merit. In this case, the apparently political objection includes a perfectly valid point: the runaway brain theory ignores male mate choice and female sexual competition, which appear to be fairly important in our species. Women are especially good at noticing

this, because they are more aware of their own competitive strategies, just as men are more aware of theirs.

The third factor that keeps the sexes similar is the mutuality of human mate choice. Both sexes are choosy when searching for longterm partners. Both compete for sexual status, both make efforts to display their attractiveness and intelligence, and both experience the elation of romantic love and the despair of heartbreak. The pure runaway theory in which males court and females choose just does not reflect the human mating game as we play it.

Evolutionary psychologists sometimes forget this because sexual selection theory is so good at predicting sex differences, and sex differences are so easy to test. As David Buss has emphasized, human sex differences are most apparent in short-term mating. Men are more motivated to have short-term sexual flings with multiple partners than women are. Women are much choosier than men in the short term. Short-term mating is exciting and sexy, but it is not necessarily where sexual selection has the greatest effect. Human females, much more than other great apes, conceal when they are ovulating. This means that a single act of short-term copulation rarely results in pregnancy. Almost all human pregnancies arise in sexual relationships that have lasted at least several months, if not years. Modern contraception has merely reinforced this effect.

Human males are generally not as choosy about short-term affairs as females. There is very little opportunity cost to short-term mating for men. It does not exclude other sexual options. But men get much choosier about medium- and long-term relationships, because their opportunity costs increase dramatically. If they are in a sexual relationship with one woman, it is very difficult to sustain a sexual relationship with another woman. They cannot give both their full attention. They must make choices—sexual

choices.

Evolutionary psychologists such as Doug Kenrick have good evidence that when it comes to choosing sexual partners for longterm relationships, men and women increase their choosiness to almost identical levels. They also converge in the features they prefer. Kenrick found that for one-night stands, women care much

more about the intelligence of their partner than men do, but for marriage, men and women have equally high standards for intelligence. For almost every sexually desirable trait that has been investigated, men and women get choosier as relationships get "more serious." For most couples, getting serious means having babies. Sexual selection works through the sexual choices that actually result in babies being born, not just the sexual choices that result in a little copulation.

Women quickly learn the difference between male short-term mating and long-term commitment. They know it is generally easy to get a man to have sex, but hard to get him to commit. Male mate choice is usually exercised not when deciding whether to copulate once, but when deciding whether to establish a long-term relationship. This is why sexual competition between women is usually competition to establish long-term relationships with desirable men, not competition to copulate with the largest number of men. Even polygynous men have limited time and energy, and so have high incentives to be choosy about their longterm partners.

It seems reasonable to assume that most human offspring throughout recent human evolution were the products of longterm sexual relationships. (By primate standards, "long term" means at least a few months of regular copulation.) In picking long-term sexual partners, our male and female ancestors both became very choosy. That choosiness is what drove sexual selection, which depends on competition to reproduce, not competition to copulate. Concealed ovulation in our female ancestors undermined the link between single acts of copulation and effective reproduction. If most human reproduction happened in longterm relationships that were formed through mutual choice, then most human sexual selection was driven by mutual choice, not just by female choice.

Mutual choice is good at producing sexual equality in courtship abilities. If men and women became equally choosy in the long-

term relationships that produced almost all babies, then men and women would have been subject to an equal degree of sexual selection. Their mental capacities for courtship would have

evolved to equally extreme degrees. Their mental capacities for sexual choice would also have evolved equally.

At first glance, mutual choice seems to offer a solution to the problems posed by the runaway brain theory. It accounts for the sexual equality of brain size and human intelligence that the simple runaway model can not explain. The only problem is that mutual choice renders traditional models of runaway sexual selection irrelevant, because runaway depends on intense choosiness by one sex and intense competition by the other. It depends on sexual asymmetry. If human sexual selection has been driven mostly by sexually symmetric mutual choice to form relatively long-term relationships, then runaway is not the right model for human mental evolution.

Assessing the Runaway Brain Theory

If one acknowledges that sexual selection has played a role in the human mind's evolution, it is crucial to understand the runaway process, even if the runaway brain theory itself does not work. The reason is that runaway sexual selection is ubiquitous. Take any population with mate choice that is not totally monogamous, and runaway will occur sooner or later, going off in some direction. Runaway is endemic in sexual selection. Like convection beneath the Sun's surface, it is always bubbling away, mixing up sexual ornaments and sexual preferences, sometimes shooting off in a random direction like a solar flare. Any species that reproduces sexually using mate choice has probably been caught up in the runaway process repeatedly.

The runaway brain theory proposes that most of our unique mental capabilities evolved through ordinary runaway sexual selection. While the theory has a number of strengths, it also, as we have seen, has a couple of crippling weaknesses. Runaway sexual selection is good at explaining traits that are extreme, striking, and costly; that are attractive to the opposite sex; and that have little apparent survival value. Some of the human mind's more puzzling capacities seem to fit this pattern: art, music, poetic language, religious beliefs, political convictions, creativity, and kindness.