The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (30 page)

The same group-protection effect would have guarded females against sexual predators. Ancestral women could protect one another from harassment and rape, just as other female primates do. From a female's point of view, a strong male partner would be a mixed blessing. He could fend off unwanted attention from other males, but he could also beat you up if he got jealous or angry. Women consistently show preferences for tall, strong males in mate choice studies, but this may reflect a preference for good genes and high fitness, rather than a preference for a male capable of physical violence and intimidation that might get turned against her or her children.

Interviews with contemporary hunter-gatherer women by anthropologists such as Marjorie Shostak reveal that these women view many men as more trouble than they're worth. If the men are hanging around, they usually eat more food than they provide, and demand more care than they give one's children. If they have very high fitness, then their good genes, good sex, and good conversation might compensate for their messiness and lethargy. But if they are only average, their potential for sexual jealousy and violent irritability may render them a net cost rather than a benefit.

On the other hand, David Buss and other evolutionary psychologists have amassed considerable evidence that modern women generally favor tall, strong, healthy, and self-confident

men, all else being equal. These traits may be favored because they would have correlated with good hunting abilities and protection abilities under ancestral conditions. However, as we shall see in the next chapter, many of these traits also reveal good genes—they are genetically heritable, and they work as effective fitness indicators. It is not yet clear whether the genetic or non-genetic benefits of such traits were more important to women. Mate choice mechanisms should evolve to capture both sorts of benefit whenever possible, so they may be difficult to disentangle.

There is still much debate about the importance of fathers in human evolution. Men show some signs of having been selected as good and helpful fathers, but our paternal instincts have not been well researched yet. Modern fathers form strong emotional attachments to their children, and this is probably an evolved propensity. A few of them even spend almost 20 percent as much time doing child care as their female partners do. Recent surveys show that Japanese fathers are starting to play with their children for almost seven minutes a day. That is a relatively high amount of paternal care compared to other male mammals. But to better understand the evolution of fathers, we need a closer look at how courtship may have overlapped with parenting.

Combining Courtship and Parenting

Before contraception, our female ancestors would have produced their first child by around age 20, within a few years of reaching sexual maturity. (Female puberty probably happened several years later in prehistory than it does now, because the modern fat-rich diet artificially hastens puberty and increases teenage fertility.) Before legally imposed monogamous marriage, individuals probably passed through several sexual relationships during their reproductive years. These two patterns imply that most courtship during most of human evolution occurred between adults who already had children by previous relationships. Without nannies, nurseries, or schools, those children would have been hanging around their mothers almost all the time. (In the wild, no primate female ever grants parental custody

of her children to their father after they split up.) Where there were women, there were usually already children. In modern Western societies we forget how parenting and courtship must have overlapped because we have children later in life, have very few of them, and exclude them from adult social life.

Female hominids must have juggled their courtship efforts with their mothering. Some of their courtship displays may have originated by turning normal motherly duties into better fitness indicators and entertainments. If they must tell stories to entertain their children, and if potential male mates are within earshot, they might as well make the stories appeal on both the child and the adult levels. If they must feed their children, and they want to attract a man, they might as well forage for something unusually tasty Male mate choice almost never had the luxury of favoring a woman who did not yet have any children, who could spend all her time frolicking and canoodling. The important variable was not whether a female already had children, but whether she was a cheerful mother or a careworn mother, a beautiful mother or an ugly mother, an intelligent mother or a boring mother. Sexual competition between females was mostly sexual competition between mothers.

Moreover, mothers probably cared about the views of their children in choosing new sexual partners, so female choice must have intertwined with children's choice. Kids who hated their mother's new boyfriend might have destroyed his chances of sustaining a successful relationship. Mothers had good reasons to listen to their children's likes and dislikes, because their children were the vehicles carrying their genes. The children were every mother's paramount concern. A healthy child in hand was worth two male lovers in the bush. This put male hominids in an unusual position: their courtship had to appeal not only to mothers but to their children. This has a surprising implication. If children's judgments influenced mate choice, then they influenced sexual selection, and children's preferences indirectly shaped the evolution of adult male humans.

So, what did those hominid kids do to us? They did not make

male humans as good at parenting as the average female mammal, but they made them better fathers than in almost any other male primate species. Men bring children food, make them toys, teach them things, and play with them. Their willingness to do this even for step-children could be viewed as a side-effect of a male adaptation for taking care of their own genetic offspring. But perhaps fatherly support and protection of step-children was the norm in the Pleistocene. If typical sexual relationships only lasted a few years, men were much more likely to be playing with some other guy's children than their own. Many evolutionary psychologists have pointed out that what looks like paternal effort may actually have evolved through sexual choice as courtship effort. Men attracted women by pleasing their kids.

This is not to say that step-fathers are all sweetness and light. Evolutionary psychologists Martin Daly and Margo Wilson have found that men in every culture are about a hundred times more likely to beat and kill their step-children than their genetic children. There are clear evolutionary reasons for that. When male lions and langur monkeys mate with a new female, they routinely try to kill all of her existing offspring. Those offspring do not carry the males' genes, so by killing them the males free the females to conceive their own offspring, who will carry their genes. The risk of infanticide by males is a big problem for many female primates. Yet is it much less of a worry for modern women. I want to highlight how kind most human step-fathers are compared with other male primate step-fathers. Not only do we consistently fail to kill our step-children like lions try to, we sometimes take reasonably good care of them. Surprisingly, human fathering instincts may have evolved through sexual selection for pleasing the existing children of potential female mates. Of course, where those existing children happen to be ours because we are still in a long-term sexual relationship, there are extra genetic incentives to be good fathers.

Where Sexual Choice Did Its Work

strategic. When we talk about their "mating pattern," this is just a generalization across a lot of individual strategic behavior. The individual sexual choices, not the aggregate mating pattern drive sexual selection. To describe our ancestors as following mating patterns like "moderate polygamy" and "serial monogamy" is just a useful shorthand for identifying these sexual selection pressures.

For sexual choice to have any evolutionary effect, different individuals must produce different numbers of surviving offspring by virtue of their sexual attractiveness. How did the most attractive hominids leave more offspring? When we focus on the polygynous aspects of ancestral mating, it is easy to see. The most attractive males simply inseminate a larger proportion of females, and the least attractive males inseminate fewer. The next generation will inherit many genes from the most attractive males, and none from the least attractive. Polgyny raises the possibility of runaway sexual selection, which is driven mostly by differences in male reproductive success. Also, polygyny helps explain sex differences. The higher variation in reproductive success among males explains why male humans are so keen to show off, to dominate culture and politics, and to broadcast indicators of their fitness to any female who might listen. To the extent that our ancestors were polygynous, there were sexual selection pressures for males to display more intensely then females.

However, we should not assume that sexual selection requires polygyny. As Darwin appreciated, the sexual choices that lead to monogamous pairs can also be crucial. Is it possible that sexual selection can produce equal mental capacities for courtship in both sexes? How can the sexual choices that create monogamous couples possibly have any evolutionary effects? Sexual selection depends on differences in reproductive success, and at first glance monogamy looks as if it produces no such differences.

Suppose that sexual choice among our hominid ancestors worked as follows. Male and female hominids both tried to attract the best sexual partner they could. If they liked that partner's company, they hung out a lot together, had a lot of sex, and produced a child. If they still liked each other after the baby

arrived, they stayed together and produced another one. If they did not, they separated and looked for the best new partner they could find. Most hominids spent most of their lives in some kind of sexual relationship with somebody Most sexual relationships longer than a few months produced at least one child.

Sexual Selection When Everyone Ends a Partner

To see how sexual selection can work even when everyone pairs up into couples, we need a thought experiment. Like all good thought experiments, it will be simplistic, unrealistic, and cartoon-like. But it will give us a surprising result. In this imaginary scenario, every hominid individual finds a sexual mate, every relationship is totally monogamous and permanent, and every relationship produces an identical number of babies. And yet, as long as sexual choice favors fitness indicators, sexual choice can still drive sexual selection by producing unequal numbers of grandchildren. Here's how it works.

Imagine a tribe of hominids, half of them male and half female, all single, all just reaching sexual maturity at the same time. Some males have higher fitness than other males, and they advertise their higher fitness using fitness indicators such as vigorous dancing, intelligent conversing, or realistic cave-painting. Some females have higher fitness than other females, which they advertise through the same sorts of fitness indicator. Fitness is genetically heritable, so higher-fitness parents generally have higher-fitness offspring. The tribe has a tradition of strict monogamy and no infidelity. Every individual has to pick a partner once and stick with them until they die. Both sexes exercise mate choice, accepting and rejecting whomever they want.

What will happen? Each individual wants to attract the highest-fitness mate they can, because they want the best genes for their offspring. There will be a sorting process. Probably, the highest-fitness male will court the highest-fitness female first. If she is sensible, she will accept him, and they will pair off, leaving the rest of the tribe to sort themselves out. The second-highest-fitness male is disappointed. He wanted the highest-fitness female, but

could not attract her. He must settle for the second-highestfitness female. She is also disappointed, because she wanted the best male. But she settles for male number two, because she cannot do any better. Perhaps they fall in love, thanking their lucky stars that they did not end up with the cold and snooty number ones, or the repulsively inferior number threes. Now the third-highest-fitness male is doubly heart-broken. Golden female number one and silver female number two have both ignored him, leaving him to court bronze female number three. He can't do any better, and neither can she, so they pair off. And so on. Eventually, the whole tribe sorts itself into mated pairs of roughly equal fitness.

The fitness matching does not result from any individual's preference for a similarly ranked mate. Instead, it results from the interaction of everyone's preferences during the sorting process. Everybody would prefer a higher-fitness mate rather than a same-fitness mate. But the opposite sex feels the same way too. For a male to mate above his fitness, a female would have to mate below her fitness. Her response to his offer will be "Dream on, loser." Likewise for females trying to mate above their fitness. Individuals have no realistic hope of mating far above their own fitness level, or any willingness to mate below their fitness.

The result will be that mated pairs will correlate highly for fitness. If height correlates with fitness, they will be of similar height. If intelligence correlates with fitness, they will be similarly bright. If facial attractiveness correlates with fitness, they will be similarly beautiful This is basically what we see in modern human couples: a fairly high degree of "assortative mating" for fitness indicators.

After the mated pairs start having sex, babies start arriving. To make this thought experiment challenging, let's look at the situation where sexual selection seems weakest, and assume that every pair has exactly the same number of babies, say four babies per pair. During most of human evolution, probably only 50 percent of infants survived to sexual maturity, so two babies surviving out of four for every two parents will keep the population size stable. The question is, which mated pairs will