The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (32 page)

would make a more interesting long-term partner? The female would be deciding the same thing: does he offer anything beyond a few orgasms and some good times?

Very Simple Rules Can Lead to Very Good Sexual Choices

How smart did our ancestors have to be to make all these complicated mate choices? A cognitive psychologist might try to construct mathematical models of how all the information about sexual cues gets integrated, and how all the individuals get compared. This makes the mate choice task look daunting. However, my research on simple rules for mate choice suggests that very good sexual choices can result from very fast, very simple decision rules.

Fitness indicators themselves make sexual choice simple. When a female long-tailed widowbird chooses a mate, she can get a pretty good estimate of his fitness simply by looking at the length and symmetry of his tail feathers. She does not need a complete DNA profile highlighting all his mutations—the tail is all she needs to see. The fitness indicators that our ancestors evolved also made sexual choice much easier. They could just pay attention to a few cues like height and facial appearance, and get a pretty good estimate of an individual's fitness. Each trait that we consider sexually attractive already summarizes a huge amount of information about an individual's genes, body, and mind.

We do not need to combine the information about these sexual traits in very complicated ways, either. It might seem difficult to compare two possible mates who differ in dozens of ways. It seems that the mathematically correct procedure would be to take each of their features, multiply it by its importance, add up all the results, and then compare the total score for each individual. But this is not necessary. Psychologist Gerd Gigerenzer and his colleagues have found that if you have to pick between two prospects based on a number of features, you can make extremely good decisions by doing something much, much simpler You can rank the features you find most important, then compare the prospects on each feature until you find a feature where one

prospect is clearly superior. For example, if you think intelligence and beauty are the most important two features in a sexual partner, you can just go down your list and compare each prospect. Is one significantly more intelligent than the other? If so, pick the bright one. If not, then is one significantly more physically attractive than the other? If so, pick the beautiful one. If not, choose randomly, because it doesn't matter. Gigerenzer's team has a lot of evidence that this very simple rule, which they call "Take the Best," makes decisions almost as good as the most sophisticated mathematical decision rules in almost every situation. It has astonishing power as a decision rule, yet it is very simple. If our ancestors used a rule of thumb like Take the Best to choose mates, they could have made very good decisions without needing to process a great deal of information using very complicated rules.

Although sexual decision-making can itself be fast and efficient, it sometimes takes time to acquire the relevant information about a potential mate. If a woman is interested in assessing a man's personality, intelligence, and experiences, it may take weeks of conversation before she has (unconsciously) gathered all the information she needs to fall in love. As we shall see in Chapter 10, conversations during courtship are how we learn the most about potential mates, and these conversations take time. Insofar as men may be satisfied with certain minimal standards of physical appearance before their sexual interest is aroused, their sexual decision-making may appear faster—but only because physical appearance can be judged much faster than character. When it comes to making long-term sexual commitments based on traits that are more than skin deep, men may take even longer than women.

Another challenge is to decide when to form a serious relationship while one is searching through a sequence of encounters and consortships. Economists and statisticians have developed mathematical models of optimal search that look appropriate. But here again a simple rule can do much better. The standard optimal search strategy is called the 37 percent rule. It is useful

when you are looking for the best candidate for a position, and you encounter the candidates one at a time, and you have to offer the position on the spot to the first candidate you like, without going back to previously interviewed candidates. This is somewhat like looking for a long-term mate. The 37 percent rule says that you should estimate how many total candidates are likely to apply for the position, interview the first 37 percent of them, and remember the best out of that initial sample. Then, keep interviewing until you find a candidate who seems even better than that. Once you find that better candidate, stop searching and stick with that one. The trouble with this rule is that the time and energy costs of searching can grow very large if you have a large number of possible candidates. For single New Yorkers, it is infeasible to date 37 percent of Manhattan's population before finding a spouse.

In our research on mate search strategies, colleague Peter Todd and I found that a rule we call "Try a Dozen" performs as well as the 37 percent rule under a wide range of conditions. Try a Dozen is simple: interview a dozen possible mates, remember the best of them, and then pick the very next prospect who is even more attractive. You do not have to estimate the total number of potential mates you will encounter in your reproductive lifetime; you only have to bet that you will meet at least fifty or so. Humans seem to follow something like the Try a Dozen rule: we get to know a number of opposite-sex friends during adolescence, fall in love at least once, remember that loved one very clearly, and tend to marry the next person who seems even more attractive. Each individual is "satisficing" looking for someone who is pretty good and good enough, rather than the absolute best they could possibly find. But at the evolutionary level, these satisficing rules impose sexual selection that is almost as strong as the most complicated, perfectionist decision strategy.

In general, very simple rules of thumb can result in sexual choices that are almost as good as the best strategies developed through mathematical analysis. Our ancestors did not have to have sexual supercomputers in their heads in order to make very

good sexual choices under Pleistocene conditions of great uncertainty, limited information, and potential deception. Sexual selection does not require a sophisticated set of sexual choice rules. What matters is how efficient the rules are at distinguishing between mates. If very simple rules can make fairly good sexual decisions, then, across many matings and many generations, those rules can impose very strong sexual selection.

Indicators for Qualities Other than Fitness

When trying to attract a sexual partner, heritable fitness is not the only thing worth advertising. When males and females cooperate to rear offspring, they should care about more than each other's good genes. They should seek mates in good health because they are more likely to survive as partners and parents. They should seek mates capable of efficient cooperation and coordination, so they make an effective team. Since health and future cooperation cannot be assessed directly, they must be estimated using indicators such as energy level and kindness. Those indicators can evolve according to the same principles as fitness indicators.

Usually, there is a lot of overlap between basic fitness and these other qualities. Condition-dependent indicators can advertise both heritable fitness and the aspects of bodily and mental condition that are important for shared parenting. An individual who is grossly incompetent at finding food may have bad genes, bad condition, and bad parenting potential.

In principle, sexual choice could sometimes put non-heritable qualities ahead of heritable fitness. If the environment is so demanding that a female simply cannot raise a child by herself, then she might favor an attentive, experienced father, even if he has a lower general fitness than a charming athletic genius who is hopelessly incompetent with babies. However, she might still prefer to have an affair with the genius and let the experienced father raise the resulting child. New DNA methods for establishing paternity have shown that this sort of eugenic cuckoldry is surprisingly common in birds previously thought to be monogamous, and in humans.

Until recently, evolutionary psychology emphasized the non-genetic benefits of mate choice. This emphasis may have come in part from sexual selection terminology favored by biologists in the 1980s. Food gifts, nests, territories, and fertility were termed the "direct" benefits of mate choice, and good genes were termed the "indirect" benefits; it sounds more secure to receive a direct than an indirect benefit. In particular, leading evolutionary psychologists such as Don Symons, David Buss, and Randy Thornhill focused on the material benefits that high-status men could offer women, and the fertility benefits that healthy young women could offer men. This has been a powerful research strategy for explaining many sex differences in human mating behavior.

However, many male human courtship behaviors that appear to give purely material benefits to females may have evolved mainly as fitness indicators. Males of many species give females food during courtship. Male scorpionflies give females the prey they have caught. Our male ancestors probably gave females a share of the meat from the hunt. Until recently, men in modern societies brought home almost all of the money necessary to sustain their families. Don't females in all cases simply want a good meal instead of good genes? I think the analogy is deceptive. Male scorpionflies give females a significant proportion of all the calories the female will need to produce her next batch of eggs. Modern men used to give women all the money they needed to live in a market economy. But the meat provided by our male ancestors may have been only a minor contribution to the energy needs of a mother and her children. A pregnant hominid would have needed about four pounds of food a day for 280 days, about a thousand pounds in total. If a male hominid gives her ten pounds of meat during a month-long courtship, that's fairly generous by modern hunter-gatherer standards, but it is less than 1 percent of the food she will need just during the pregnancy.

Of course, given a choice between a fitness indicator that offers zero material benefits (such as an impressive courtship dance) and one that happens to produce a material benefit (such as an impressive hunting success), evolution may favor females who

appreciate the material benefit. From a fitness indicator viewpoint, the material benefits simply bias evolution to favor fitness indicators that happen to deliver practical benefits in addition to information about mutation load.

Likewise, male defense of good territories may have evolved as a fitness indicator as well as a material benefit. Generally, female animals forage where they want, exploiting the available food resources. Males follow the females around and try to mate with them. The strongest males often succeed in driving the weakest away from the prime food-patches where the females have already decided to forage. Since the females might as well prefer a stronger to a weaker male, they might as well mate with the male who happens to be defending their food-patch. To a human observer used to the idea of land ownership, it might look as if the strong male has "acquired ownership" of the territory, which he generously allows the females to use. Perhaps even in the male animal's mind, he "owns" the territory. But to the females, they are just foraging wherever they want. The males may be running around and fighting each other, and large, muscular males may happen to last longer and stay closer to the females. The females have little incentive to go chasing after the smaller, weaker males that were driven away, so they may tend to mate with the stronger males. The females thus use the male's ability to defend the territory from other males as a fitness indicator. Sometimes the strategies of sexual choice are so efficient that they hardly look like active sexual choice at all. As long as the females do not stumble across any male trait that is a better fitness indicator than resource-defense ability, it may look as if the male automatically wins "the right to mate" by "owning the territory." But that would be missing the point. The females may be using the cue of resource-defense ability mainly to get good genes, not to get food.

In modern market economies people put a high value on wealth indicators during courtship. This can be rational, given the range of goods and services that money can buy, and the difference it can make to one's quality of life. As Thorstein Veblen argued a century ago, modern culture is basically a system of conspicuous

consumption in which people demonstrate their wealth by wasting it on luxuries. Wealth indicators follow the handicap principle just as fitness indicators do, but this makes it easy to mistake one for the other. David Buss has amassed a lot of evidence that human females across many cultures tend to prefer males who have high social status, good income, ambition, intelligence, and energy— contrary to the views of some cultural anthropologists, who assume that people vary capriciously in their sexual preferences across different cultures. He interpreted this as evidence that women evolved to prefer good providers who could support their families by acquiring and defending resources. I respect his data enormously, but disagree with his interpretation.

The traits women prefer are certainly correlated with male abilities to provide material benefits, but they are also correlated with heritable fitness. If the same traits can work both as fitness indicators and as wealth indicators, so much the better. The problem comes when we try to project wealth indicators back into a Pleistocene past when money did not exist, when status did not imply wealth, and when bands did not stay in one place long enough to defend piles of resources. Ancestral women may have preferred intelligent, energetic men for their ability to hunt more effectively and provide their children with more meat. But I would suggest it was much more important that intelligent men tended to produce intelligent, energetic children more likely to survive and reproduce, whether or not their father stayed around. In other words, I think that evolutionary psychology has put too much emphasis on male resources instead of male fitness in explaining women's sexual preferences.