The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (21 page)

The fitness indicator does not just recruit an increased share of an organism's energy: it also makes itself dependent on an increased proportion of an organism's genes. Rowe and Houle call this process "genic capture." The indicator captures a larger amount of information about an individual's genetic quality. Typically, this might work by a trait evolving a little bit more complexity, recruiting some of the genes that influence growth and development processes already evolved for other adaptations. This genic capture process makes the fitness indicator a window

on an animal's genome. As the window grows wider through genie capture, the indicator lets an observer see a larger amount of all the genetic variation in fitness with the population, making it easier to choose mates for their good genes. Good fitness indicators give sexual choice a panoramic view of a potential mate's genetic quality.

It is not clear yet exactly how genic capture works, and this feature of Rowe and Houle's model needs further research. If it does work, and if the human brain's complexity evolved in part through genic capture, then there is an interesting implication. It would explain why so many unique human mental abilities look to some biologists like "spandrels," mere side-effects of other adaptations. Stephen Jay Gould has argued that most of our uniquely human capacities did not evolve for specific adaptive functions, but emerged as side-effects of already-existing brain circuits and learning abilities. Like most evolutionary psychologists, I find that argument weak for many reasons—for example, it fails to explain why other large-brained species such as dolphins, whales, and elephants did not invent paleontology or socialism.

However, Gould's argument may have this grain of truth: the human brain's distinctive power is its ability to advertise a lot of the computational abilities that were already latent in the brains of other great apes. This does not mean that music, art, and language came for free just because an ape brain tripled in size. But it might mean that when sexual selection seized upon the ape brain as a set of possible fitness indicators, the genic capture process recruited a lot of pre-existing brain circuitry into human courtship behavior. It made that brain circuitry more manifest in courtship behavior, more condition-dependent, and more subject to sexual choice. Our brains may look like a set of spandrels, but they look that way only because our mental fitness indicators are so efficient at advertising the brain's many abilities. (Of course, fitness indicators are different from spandrels because they evolved through sexual selection to have a specific courtship function, whereas spandrels, by definition, do not have any specific evolved function.)

Mental Traits as Fitness Indicators

Fitness indicator theories like Rowe and Houle's model can help us to understand the evolution of the human mind. Our capacities for music, art, creativity, humor, and poetry do not look like ordinary adaptations are supposed to look. Evolutionary psychologists like John Tooby, Leda Cosmides, David Buss, and Steven Pinker have developed some rules for recognizing mental adaptations. If a human mental trait evolved through natural selection for some specific function, it is supposed to show small differences between people, because selection should have eliminated maladaptive variation long ago. It is supposed to show low heritability, because selection should have eliminated all genes other than the optimal ones long ago. It is supposed to be efficient and low in cost, because natural selection favors efficient problem-solving. And it is supposed to be modular and specialized for solving a particular problem, because modular specialization is the efficient way to engineer things.

Fitness indicators violate all these criteria. If a mental trait evolved through sexual selection as a fitness indicator, it should show large differences between people. It evolved specifically to help sexual choice discriminate in favor of its possessor at the expense of sexual rivals. Fitness indicators can show high heritability because they tap into genetic variation in fitness, and fitness usually remains heritable. For fitness indicators to be reliable, they have to be wasteful, not efficient. They have to have high costs that make them look very inefficient compared with survival adaptations. Finally, fitness indicators cannot be totally modular and separate from other adaptations, because their whole point is to capture general features of an organism's health, fertility, intelligence, and fitness. The peacock's tail appears to fit this profile as a fitness indicator, and many human mental abilities do as well.

To traditional evolutionary psychologists, human abilities like music, humor, and creativity do not look like adaptations because they look too variable, too heritable, too wasteful, and not very modular. But these are precisely the features we should expect of fitness indicators. If a human mental trait shows large individual

differences, high heritability, high condition-dependence, high costs, and high correlations with other mental and physical abilities, then it may have evolved through sexual selection as a fitness indicator.

If we make an inventory of what the human brain can do, we find two general themes: very few of the ancient mental abilities that we share with other apes look like fitness indicators, but many mental abilities unique to humans do look like fitness indicators. There are probably thousands of psychological adaptations in the human mind. The vast majority are shared with other species. Some evolved hundreds of millions of years ago and are shared with thousands of species. Some evolved only a few million years ago and are shared only with other great apes. We have exquisitely efficient mechanisms for regulating our breathing, controlling our limbs, keeping our balance, seeing colors, remembering spatial locations, learning foraging skills, being kind to offspring, feeling pain when injured, remembering faces, making friends, punishing cheats, perceiving social status, estimating risks, and so forth. Steven Pinker has explored many of these mechanisms in his book
How the Mind Works.
When I propose a shorthand slogan like "the human mind evolved through sexual selection," I do not mean that sexual selection shaped all of these adaptations that we share with other primates. Of course, about 90 percent of our psychological adaptations evolved through standard natural selection and social selection to solve routine problems of surviving and living in groups. Evolutionary psychology has proven very good at analyzing these adaptations.

My interest is in the psychological adaptations that are uniquely human, the 10 percent or so of the brain's capacities that are not shared with other apes. This is where we find puzzling abilities like creative intelligence and complex language that show these great individual differences, these ridiculously high heritabilities, and these absurd wastes of time, energy, and effort. To accept these abilities as legitimate biological adaptations worthy of study, evolutionary psychology must broaden its view of what an adaptation should look like. At the moment, too many scientists

are mis-describing effective fitness indicators like music and art as if they were nothing more than cultural inventions or learned skills. Their expression certainly depends on cultural traditions and years of practice, but other species with different genes cannot learn to do them no matter how hard they might try. If one banishes all these fitness indicators to the realm of "culture," then it does not look as if sexual choice had much impact on the human mind's evolution. But if one accepts fitness indicators as legitimate biological adaptations, then one starts to see the tracks of sexual selection all over our minds.

The Hominid That Wasted Its Brain

To sum up the last few sections, I think that the handicap principle casts a new light on the human brain. Everyone who proposes a theory about the brain's evolution mentions its costs. Our brains are only 2 percent of our body weight, but they consume 15 percent of our oxygen intake, 25 percent of our metabolic energy, and 40 percent of our blood glucose. When we spend several hours thinking really hard, or just conversing with people whose opinion matters to us, we get hungry and tired. Our brains cost a lot of energy and effort to run. Usually, theorists argue that these costs must have been balanced by some really large survival benefits, otherwise the brain could not have evolved to be so large and costly. But that survivalist argument holds only as long as one ignores sexual selection.

If we view the human brain as a set of sexually selected fitness indicators, its high costs are no accident. They axe the whole point. The brain's costs are what make it a good fitness indicator. Sexual selection made our brains wasteful, if not wasted: it transformed a small, efficient ape-style brain into a huge, energy-hungry handicap spewing out luxury behaviors like conversation, music, and art. These behaviors may look as if they must be conveying some useful information from one mind to another. But from a biological viewpoint they might signify nothing more than our fitness, to those who might be considering merging their genes with ours.

The better our ancestors become at articulating their thoughts, the deeper the principles of wasteful sexual signaling could reach into their minds. By favoring fitness indicators, sexual choice demanded courtship behavior that stretched the mind's capacities. It demanded that which is difficult. It forced the human brain to evolve ever greater condition-dependence, and ever greater sensitivity to harmful mutations. It asked not what a brain can do for its owner, but what fitness information about the owner a brain can reveal.

Are Fitness Indicators Immoral?

The idea that the human mind evolved as a bundle of fitness indicators does not sit comfortably with contemporary views of human nature and human society. In fact, it violates at least eight core values commonly accepted in modern society. Variation in fitness betrays our belief in human equality. The heritability of fitness violates our assumption that social and family environments shape most of human development. Loudly advertising one's fitness violates our values of humility, decorum, and tact. Sexual status hierarchies based on fitness violate our belief in egalitarian social organization. The idea that people sort themselves into sexual pairs by assessing each other's fitness violates our romantic ideal of personal compatibility. The conspicuous waste demanded by the handicap principle violates our values of frugality, simplicity, and efficiency. The sexual choice mechanisms that judge individuals by their fitness indicators violate our belief that people should be judged by their character, not the quality of their genes. Finally, it seems nihilistic to propose that our capacities for language, art, and music evolved to proclaim just one message that has been repeated loudly and insistently for thousands of generations: "I am fit, my genes are good, mate with me." A mind evolved as a set of fitness indicators can sound like a

fascist nightmare.

How is it possible for one biological concept to affront so many of our fundamental values? It seems quite astounding that a scientific idea should so consistently fell on the wrong side of the

ideological fence. I think it is no coincidence. Look at it this way; our human norms and values developed as reactions to patterns of natural human behavior that we decided should be discouraged. If a great deal of human behavior consists of advertising one's fitness, and if many ways of doing that impose social costs on others, and if moral norms develop to minimize social costs, then a lot of moral norms should be aimed directly against the irresponsible use of fitness indicators. We value humility precisely because many people are unbearable braggarts who try to flaunt their fitness indicators so relentlessly that we cannot hold a decent conversation. We value frugality because so many people embarrass everyone with their ostentatious displays of luxuries, and waste limited resources that others need. We value egalitarianism because it protects the majority from aspiring despots intent on power and polygyny

These norms do not just fall randomly from the sky. They emerged as moral instincts and cultural inventions to combat the excesses of sexual self-advertisement and sexual competition. Our moral aversion to fitness indicators may tempt us to reject them as an important part of sexual selection. But if we reject them, then it is hard to see how our moral norms evolved in the first place. It is possible, perhaps even necessary, to admit that much of human behavior evolved to advertise fitness, while simultaneously realizing that the essence of wisdom and morality is not to take our fitness indicators too seriously. This is not to say that our capacities for wisdom and morality are cultural inventions that liberate us from the imperatives of our genes. Our moral instincts may be just another set of evolved adaptations. It is not a question of "us" overriding our genetic predispositions, but of using one set of predispositions to overrule others—just as our evolved desire to preserve our looks can override our evolved taste for fat and sugar.

Another response to such worries is to point out that practically every theory of human mental evolution sounds like a fascist nightmare when we compare it with our comfortable modern lives and our political ideals. According to the Machiavellian intelligence theory, our minds evolved to lie, cheat, steal, and

deceive one another, and the most cunning psychopaths became our ancestors by denying food, territory, and sexual partners to kinder, gender souls. Richard Alexander's group warfare theory suggests that our minds evolved through genocidal violence, with larger-brained ancestors killing off smaller-brained competitors. The theory that human genes and human cultures co-evolved sounds slightly less bloody in the abstract, but it sounds that way only because it fails to specify any selection pressures that could have actually shaped anything. In terms of survival selection, what it boils down to is the view that those with brighter brains learned better technologies to grab resources before those with dimmer brains could, leaving the dimmer brains to starve, die of infectious disease, or be eaten by predators.