The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (3 page)

Of course, sexual selection through mate choice cannot favor what its agents cannot perceive. If animals cannot see the shapes of one another's heart ventricles, then heart ventricles cannot be directly shaped by sexual selection—vivisection is not a practical method for choosing a sexual partner. A major theme of this book is that before language evolved, our ancestors could not easily perceive one another's thoughts, but once language had arrived, thought itself became subject to sexual selection. Through language, and other new forms of expression such as art and music, our ancestors could act more like psychologists—in addition to acting like beauty contest judges—when choosing mates. During human evolution, sexual selection seems to have shifted its primary target from body to mind.

This book argues that we were neither created by an omniscient deity, nor did we evolve by blind, dumb natural selection. Rather, our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could. We have inherited both their sexual tastes for warm, witty, creative, intelligent, generous companions, and some of these traits that they preferred. We are the outcome of their million-year-long genetic engineering experiment in which their sexual choices did the genetic screening

Giving so much credit to sexual choice can make sexual selection sound almost too powerful. If sexual selection can act on any trait that we can notice in other individuals, it can potentially explain any aspect of human nature that scientists can notice too. Sexual selection's reach seems to extend as far as psychology's subject matter. So be it. Scientists don't have to play fair against nature. Physics is full of indecently powerful theories, such as Newton's laws of motion and Einstein's theory of general relativity. Darwin gave biology two equally potent theories: natural selection and sexual selection. In principle, his two theories explain the origins of all organic complexity functionality diversity and beauty in the universe. Psychologists generally believe that so far they have no theories of comparable power. But sexual selection can also be viewed as a psychological theory, because sexual choice and courtship are psychological activities. Psychologists are free to use sexual selection theory just where it is most needed: to explain mental abilities that look too excessive and expensive to have evolved for survival.

This sexual choice view also sounds rather circular as an explanation of human mental evolution. It puts the mind in an unusual position, as both selector and selectee in its own evolution. If the human mind catalyzed its own evolution through mate choice, it sounds as though our brains pulled themselves up by their own bootstraps. However, most positive-feedback processes look rather circular, and a positive-feedback process such as sexual selection may be just what we need to explain unique, highly elaborated adaptations like the human mind. Many theorists have accepted that some sort of positive-feedback process is probably required to explain why the human brain evolved to be so large so quickly. Sexual selection, especially a process called runaway sexual selection, is the best-established example of a positive-feedback process in evolution.

Positive-feedback systems are very sensitive to initial conditions. Often, they are so sensitive that their outcome is unpredictable. For example, take two apparently identical populations, let them undergo sexual selection for many

generations, and they will probably end up looking very different. Take two initially indistinguishable populations of toucans, let them choose their sexual partners over a thousand generations, and they will evolve beaks with very different colors patterns, and shapes. Take two populations of primates, and they will evolve different hairstyles. Take two populations of hominids (bipedal apes), and one may evolve into us, and the other into Neanderthals. Sexual selection's positive-feedback dynamics make it hard to predict what will happen next in evolution, but they do make it easy to explain why one population happened to evolve a bizarre ornament that another similar population did not.

Sexual Selection and Other Forms of Social Selection

In the 1990s evolutionary psychologists reached a consensus that human intelligence evolved largely in response to social rather than ecological or technological challenges. Some primate researchers have suggested that the transition from monkey brains to ape brains was driven by selection for "Machiavellian intelligence" to outsmart, deceive, and manipulate one's social competitors. Anthropologist Robin Dunbar has suggested that large primate brains evolved to cope with large numbers of primate social relationships. He views human language, especially gossip, as an extension of primate grooming behavior. Many researchers have suggested that acquiring our ability to attribute beliefs and desires to others, which they call our "Theory of Mind," was a key stage in human evolution.

Scientists became excited about social competition because they realized that it could have become an endless arms race, requiring ever more sophisticated minds to understand and influence the minds of others. An arms race for social intelligence looks a promising way to explain the human brain's rapid expansion and the human mind's rapid evolution.

The human mind is clearly socially oriented, and it seems likely that it evolved through some sort of social selection. But what kind of social selection, exactly? Sexual selection is the best-

understood, most powerful, most creative, most direct, and most fundamental form of social selection. From an evolutionary perspective, social competition centers around reproduction. Animals compete socially to acquire the food, territory, alliances, and status that lead to reproduction. Sexual selection is the most direct form of social selection because mate choice directly favors some traits over others, and immediately produces offspring that are likely to inherit the desired traits.

In other forms of social selection, the link between behavior and reproduction is much less direct. For example, the ability to form and maintain social alliances leads to easier foraging, better protection against predators, and better sexual access to desired mates. This in turn may lead to higher reproductive success, if the desired mates are willing. Other forms of social selection are important, but mostly because they change the social scenery behind sexual selection. Social selection is like the political tension between the Montagues and Capulets. It matters largely because it influences the sexual prospects of Romeo and Juliet.

Sexual selection is the premier example of social selection, and courtship is the premier example of social behavior. Theories of human evolution through social selection without explicit attention to sexual selection are like dramas without romance. Prehistoric social competition was not like a power struggle between crafty Chinese eunuchs or horticulturally competitive nuns: it was a complex social game in which real males and real females played for real sexual stakes. They played sometimes with homicidal or rapacious violence, and sometimes with Machiavellian strategizing, but more often with forms of psychological warfare never before seen in the natural world: conversation, charm, and wit.

What Makes Sexually Selected Traits So Special?

Apart from sexual selection being a special sort of evolutionary process, the adaptations that it creates also tend to show some special features. Adaptations for courtship are usually highly developed in sexually mature adults but not in youth. They are

usually displayed more conspicuously and noisily by males than by females. They produce sights and sounds that prove attractive to the opposite sex. They often reveal an animal's fitness by being difficult to produce if the animal is sick, starving, injured, or full of harmful mutations. They show conspicuous differences between individuals, and those differences are often genetically heritable. ("Heritable" implies that some proportion of the differences between individuals in a particular trait are due to genetic differences between individuals.) As we shall see, the human mind's most distinctive features, such as our capacities for language, art, music, ideology, humor, and creative intelligence, fit these criteria quite well.

However, traits with these features are sometimes not considered legitimate biological adaptations. Evolutionary psychologists Steven Pinker and John Tooby have argued that our science should focus on human universals that have been optimized by evolution, no longer showing any significant differences between individuals, or any genetic heritability in those differences. That is a good rule of thumb for identifying survival adaptations. But, as we shall see, it rules out all sexually selected adaptations that evolved specifically to advertise individual differences in health, intelligence, and fitness during courtship. Sexual selection tends to amplify individual differences in traits so that they can be easily judged during mate choice. It also makes some courtship behaviors so costly and difficult that less capable individuals may not bother to produce them at all. For art to qualify as an evolved human adaptation, not everyone has to produce art, and not everyone has to show the same artistic ability. On the contrary, if artistic ability were uniform and universal, our ancestors could not have used it as a criterion for picking sexual partners. As we shall see, the same reasoning may explain why people show such wide variation in their intelligence, language abilities, and moral behavior.

While sexually selected adaptations can be distinguished from survival adaptations from the outside, they may not feel any different from the inside. In particular, they may not feel very

sexual when we're using them. Sexual selection is a theory of evolutionary function, not a theory of subconscious motivation. When I argue that a particular human ability evolved to attract sexual partners, I am not claiming that there is some sort of Freudian sex drive at work behind the scenes. Peacock tails do not need a sexual subconscious in order to be sexually attractive, and neither do our instincts for art, generosity, or creativity.

Why Now?

If sexual selection is so great, why hasn't it been used before now to explain the most distinctive aspects of human nature? In the next chapter, I trace the reasons why sexual selection theory was neglected for a century after Darwin and why it was revived only in the 1980s. The century of neglect is important to appreciate, because virtually all of 20th-century science has tried to explain human mental evolution using natural selection alone. Even now, sexual selection is usually invoked only to explain the differences between women and men, not those between humans and other primates. Although evolutionary biologists and evolutionary psychologists all know about sexual selection, its power, subtlety, and promise for explaining human mental traits have been overlooked.

The idea that sexual choice was an important factor in the human mind's evolution may sound radical, but it is firmly grounded in current biology. Twenty years ago, this book could not have been written. Only since then have scientists come to realize how profoundly mate choice influences evolution. There has been a renaissance of interest in sexual selection, with an outpouring of new facts and ideas. Today, the world's leading biology journals are dominated by technical papers on sexual selection theory and experiments on how animals choose their mates. But this has been a secret renaissance, hidden from most areas of psychology and the humanities, and largely unrecognized by the general public.

Prudery has also marginalized sexual selection—which is, after all, about sex. Many people, especially scientists, are ambivalent

about sex: fascinated but embarrassed, obsessed yet guilty, alternately ribald and puritanical. Scientists still feel awkward teaching sexual selection to students, talking about it with journalists, and writing about it for the public. Science is not so different from popular culture in this respect. Just as there are very few good films that explicitly show sexual penetration, there have been very few good theories of human mental evolution that depict our ancestors as fully sexual beings capable of intelligent mate choice.

The sexual choice idea is also timely because it counters the charge that evolutionary psychology is some sort of "biological reductionism" or "genetic determinism." Many critics allege that evolutionary psychology tries to reduce psychology to biology, by explaining the mind's intricacies in terms of the brute replication of genes. In general, there is nothing wrong with reductionism—it is a powerful and successful strategy for understanding the world, and a cornerstone of the scientific method. However, there are serious problems with biological reductionism in the sense of trying to account for all of human nature in terms of the survival of the fittest. Often this strategy has led scientists to dismiss far too glibly many important human phenomena, such as creativity, charity, and the arts. This book tries very hard to avoid that particular type of reductionism. My theory suggests that our most cherished abilities were favored by the most sophisticated minds ever to have emerged on our planet before modern humans: the minds of our ancestors. It doesn't reduce psychology to biology, but sees psychology as a driving force in biological evolution. It portrays our ancestors' minds as both products and consumers evolving in the free market of sexual choice. My metaphors for explaining this theory will come more from marketing, advertising, and the entertainment industry than from physics or genetics. This is probably the least reductionistic theory of the mind's evolution one could hope for that is consistent with modern biology.

The Gang of Three

It began as an attempt to solve three basic problems concerning human mental evolution. These problems crop up as soon as we ask why we evolved certain abilities that other species did not evolve.