The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (17 page)

Extreme vulnerability to mutation sounds like something that natural selection could not possibly favor. Precisely. It is what sexual selection through mate choice favors. Once sexual choice seized upon the brain as a possible fitness indicator, the brain was helpless to resist. Any individuals who did not reveal their fitness

through their courtship behavior were not chosen as sexual partners. Their small, efficient, ironclad, risk-averse, mutation-proof brains died out with them. In their place evolved our sort of brain: huge, costly, vulnerable, revealing.

Our species was not the first to stumble upon the fact that complex behaviors make good fitness indicators. Songbirds reveal their fitness by repeating complicated, melodious songs. Fruitflies do little dances in front of one another to reveal their genetic quality Bowerbirds construct large mating huts ornamented with flowers, fruits, shells, and butterfly wings, presumably to reveal their quality. In fact, many species appear to use their courtship behaviors as fitness indicators. The distinctive thing about humans is that our courtship behavior reveals so much more of our minds. Art reveals our visual aesthetics. Conversation reveals our personality and intelligence. By opening up our brains as advertisements for our fitness, we discovered whole new classes of fitness indicators, like generosity and creativity.

To suggest that a mental capacity like human creative intelligence evolved as a fitness indicator is not just to throw another possible function into the arena of human evolution theories. This is not a function like hunting, toolmaking, or socializing that contributes directly to fitness by promoting survival and reproduction. Instead, fitness indicators serve a sort of meta-function. They sit on top of other adaptations, proclaiming their virtues. Fitness indicators are to ordinary adaptations what literary agents are to authors, or what advertisements are to products. Of course, they are adaptations in their own right, just as literary agents are people too, and just as advertisements are also products—the products of advertising firms. But fitness indicators work differently. They take long vacations. They are social and sales-oriented. They five in the semiotic space of symbolism and strategic deal-making, not in the gritty world of factory production. The healthy brain theory proposes that our minds are clusters of fitness indicators: persuasive salesmen like art, music, and humor, that do their best work in courtship, where the most important deals are made.

We should not expect sexually selected fitness indicators to look very useful if they are evaluated by traditional survival-ofthe-fittest criteria. They do not help animals find food or avoid predators. They do not remove parasites or feed offspring. They look costly and useless. They appear luxuriously superfluous, often resembling a pathological side-effect of something more useful and sensible. But these are precisely the features of many human mental abilities that have puzzled scientists. Art and morality look like evolutionary luxuries. Creative intelligence and language seem useful in moderation, but humans do not have them in moderation—we have them in luxuriant excess.

The idea of mental fitness indicators fills an important gap in evolutionary psychology. Physical fitness indicators form a standard part of sexual selection theory and are covered in every good evolutionary textbook. Researchers such as Randy Thornhill, Steven Gangestad, David Perrett, Anders Moller, and Karl Grammer have analyzed many aspects of the human face and body as fitness indicators that reveal health, fertility, and youth. Most evolutionary psychologists agree that human mate choice is even more focused on mind than on body, concerned as it is with assessing a person's social status, intelligence, kindness, reliability, and other psychological traits. Yet evolutionary psychology has paid very little attention to the possibility that many of our psychological traits may have evolved as fitness indicators too. The idea is not assessed in Steven Pinker's
How the Mind Works,
David Buss's textbook
Evolutionary Psychology,
or any other major work on evolutionary psychology In most such works natural selection is used to explain most of the mind's adaptations. Where sexual selection is invoked, it is almost always to explain how our mechanisms for mate choice evolved, or how some basic sex differences in sexual strategies evolved. The idea of sexual selection for mental fitness indicators has yet to be adequately explored.

To understand how these parts of the mind may have evolved as fitness indicators, we have to understand a bit more about what fitness means, why fitness varies enough to be worth worrying

about in mate choice, and what makes a good fitness indicator. After we have these principles under our belts, we can have another look at the healthy brain theory.

Evolutionary Fitness and Physical Fitness

Fitness indicators are supposed to reveal fitness—but what does "fitness" really mean? For biologists, fitness means an organism's propensity to survive and reproduce in a particular environment. Fitness in this evolutionary sense has three important features: it is relative to competitors in a species, it is relative to an environment, and it is a statistical propensity rather than an achieved outcome.

Evolutionary fitness is always relative to a population of competitors within a species. "High fitness" for a barnacle, a mayfly, an oak tree, and a human depend on very different traits, and suggest very different numbers of offspring. What ties together fitness across species is the link between fitness and evolutionary change. Genes underlying high fitness will tend to spread through a population, replacing genes for low fitness. Evolution increases fitness, by definition. In this sense, evolution is progressive: when sexual selection favors fitness indicators, it necessarily increases fitness and contributes to evolutionary progress.

Evolutionary fitness is also relative to environment. It depends on the fit between an organism's traits and an environment's features, which is why it is called "fitness" rather than "quality" or "perfection." The
Alien
films notwithstanding, there is no such thing as a super-organism that could survive and reproduce in every possible environment. When biologists talk about an organism's fitness, they usually assume that the organism's performance is being measured in an environment similar to that in which the species has been evolving for many generations. An organism that shows high fitness in an ancestrally normal environment will not necessarily show high fitness in a novel environment.

Fitness as a propensity is the most slippery concept to grasp. Fitness as I use the term is a statistical propensity, an expectation

that allows us to predict how an individual will probably fare. We attribute propensities all the time to other people: intelligence, kindness, irritability. Like fitness, these traits must be inferred rather than directly perceived. Like fitness, they allow us to make predictions that work on average over the long term, but those predictions are sometimes overridden by situational factors. Fitness is something we attribute to organisms to explain why they survive and reproduce better than their competitors. It is not just a measure of whether they do in fact survive and reproduce, because accidents can happen. A highly fit organism that we expect to thrive may be hit by lightning, or rejected as a sexual partner through some kind of situation-comedy mix-up. These failures to live up to one's fitness do not imply that the concept of fitness is vacuous. Intelligent people sometimes make errors in mental calculations, but that does not invalidate the concept of intelligence. Not all philosophers of biology agree on this propensity idea of fitness, but most do, and so do I.

In other contexts, fitness means something different. "Fitness centers" do not usually contain biologists scribbling down evolutionary equations. Instead, they are frequented by people trying to get fit, to improve their physical fitness. Fitness in the physical sense implies health, youth, athletic ability, and physical attractiveness. When George Bush appointed Arnold Schwarzenegger to head the President's Council on Physical Fitness in the early 1990s, he did not expect Schwarzenegger to improve the quality of the American gene pool. He expected him to get Americans in better shape.

Physical fitness is not relative to a population or an environment, but is relative to a norm of optimal efficiency for a body of a particular species. When we say a man is physically fit, we do not mean he is merely less fat, weak, stiff, and breathless than his peers. A whole population might be physically unfit. To be physically fit is to have a body near the peak of its potential performance, objectively efficient at turning oxygen and food into muscle power and speed. Physical fitness in this sense could even be compared across species. One could say "She is as fit as a

champion greyhound." That may be faint praise, but it is not meaningless.

Physical fitness is still environment-relative in the sense that a fit human could not thrive on a neutron star with gravity a billion times stronger than the Earth's. Yet, within the normal operating parameters of a species, physical fitness is useful across a range of situations. An athlete who is fir enough to climb Mount Everest is probably fit enough to scuba-dive, or to fly a rocket to Mars. Physical fitness manifest in one situation usually transfers fairly well to other situations. This is why triathlons and decathlons exist—there are some tradeoffs between the optimal body for distance running and the optimal body for swimming, but some individuals can be better at both than almost anyone else is at either.

Another contrast to evolutionary fitness is that physical fitness is

closer to a measurable achievement man a statistical propensity. It is less abstract, and closer to real behavioral outcomes. We expect

strength to be manifest in the consistent ability to lift heavy things. We expect aerobic fitness to be manifest in the ability to climb

stairs without losing one's breath. Accidents can still keep the fittest athlete from winning a gold medal, but the correlation between physical fitness and physical performance is usually rather high. This is why manifest physical performance is such a

good indicator of physical fitness.

Apart from physical fitness, one might also speak of "mental fitness," implying sanity, intelligence, rationality, and communication ability—as when a witness is fit to testify in court. Mental fitness shares most of the important features of physical fitness: it is relative to a norm of optimal psychological efficiency in a particular species, it is fairly general across psychological tasks, and we expect it to be manifest in real behavior. Indeed, what intelligence researchers call "general cognitive ability" or "the
g
factor" could be construed as mental fitness.

Biology students are often taught to make a very clear distinction between evolutionary fitness and physical fitness, to keep them separated by the social Atlantic that keeps professional athletes from mixing with scientists. This distinction is important

in teaching biology students to think in flexible, abstract ways about evolutionary fitness. It reminds us that evolutionary fitness is always a matter of trade-offs, or finding the optimal allocation of resources between competing demands. Physical strength is not synonymous with evolutionary fitness, because investing in larger muscles may often produce fewer offspring than investing in larger testicles, fat reserves, or brains. But the distinction makes it hard to develop good intuitions about fitness indicators, which tend to advertise fitness in both the evolutionary and the physical sense.

The Oxford biologist W. D. Hamilton has reminded his colleagues that, within a given species, physical fitness is often rather tightly linked to evolutionary fitness. In his work on sexual selection he has tried to revive a more intuitive concept of fitness in which survival and reproduction do depend on basic physical variables like health, strength, energy, and disease-resistance. Within a species, healthier, stronger animals do tend to survive better, reproduce better, and attract more mates. This correlation between evolutionary fitness and physical (or mental) fitness keeps "the survival of the fittest" from being a tautology.

Evolutionary fitness is linked to physical and mental fitness by something that biologists call "condition." In fact, an animal's "condition" is basically its physical fitness, health, and energy level, A high-fitness animal may be in poor condition due to a temporary injury or food shortage. A low-fitness animal might be in good condition due to a zoo taking very good care of it. In a science laboratory, we can disentangle condition from fitness. We can randomly assign different diets to different animals, or infect an experimental group with a communicable disease and protect a control group from that disease. But in nature, animals largely determine their own condition through their own efforts. The abilities to find food, resist disease, and avoid parasites are major determinants of condition, and major components of fitness. In nature, fitness generally correlates with condition. Good condition is thus a pretty good indicator of high fitness.

Of course, there may be droughts, disasters, food shortages,

and epidemics, when all members of a population suffer from poor condition. But even then, higher-fitness animals may suffer less than lower-fitness animals do. The correlation between fitness and condition may remain, despite fluctuations in a population's average condition. In fact, fitness may sometimes be easier to assess under challenging conditions because individual differences in ability may then become more apparent. This is why romantic novels include adventure and risk: emergencies bring out the best in heroes and the worst in pretenders.