The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (14 page)

In summary, the overall evidence for sex differences is confusingly mixed. At the level of brain size and raw intelligence, human sex differences are too small for the runaway brain theory to work. Although brain size within each sex is correlated about 40 percent with general intelligence, the slightly larger brains of males do not yield a higher general intelligence than those of females. At the level of sexual behavior and cultural output, sex differences are enormous, but they are shrinking rapidly, and are

conflated with patriarchal cultural traditions. Overall, this pattern of evidence does not support a strong version of the runaway brain theory, nor does it support any other theory in which male sexual competition through toolmaking, hunting, or group warfare was the driving force behind the human mind's evolution. If sexual selection was important in the mind's evolution, it could not have been a type of sexual selection that produces large sex differences in brain size or general intelligence. At this point, it may help to step back from the runaway brain theory and consider sex differences in a more general evolutionary framework.

Eggs and Sperm

Sexual selection demands sexual reproduction, but it does not demand distinct sexes. If hermaphrodites exercise mate choice, they can evolve sexual ornaments. A small number of animals and a large number of flowering plants are hermaphroditic. Because they still compete to attract mates, they still evolve sexual ornaments. Sexual selection does not require sex differences, and does not always produce sex differences.

However, in most animals, distinct sexes have evolved. They simply specialize in making DNA packets of different sizes. The

female sex evolved to make large packets in which their DNA comes with additional nutrients to give offspring a jump-start to their development. The male sex evolved to make the smallest possible packets in which their DNA is almost naked, contributing no nutrients to their offspring. Females make eggs; males make sperm. The fundamental sex difference is that females invest more nutrient energy in offspring than males.

In the early 1970s, biologist Robert Trivers realized that, from this difference in "parental investment," all else follows. Because eggs cost more for females to make than sperm costs for males, females make fewer eggs than males make sperm. But since each offspring requires only one of each, the rarer type of DNA packet, the egg, becomes the limiting resource. Thus, Trivers argued, it makes sense that males should compete more intensely to fertilize

eggs than females do to acquire sperm, and that females should be choosier than males. Males compete for quantity of females, and females compete for quality of males. Trivers' supply-anddemand logic explained why in most species, males court and females choose.

In female mammals the costs of pregnancy and milk production are especially high, amplifying the difference between male competitiveness and female choosiness. For example, the minimum investment human female ancestors could have made in their offspring would have been a nine-month pregnancy followed by at least a couple of years of breast-feeding. The minimum investment our male ancestors could have made in their offspring would have been a few minutes of copulation and a teaspoonful of semen. (For most male primates, that is not only the minimum, but the average.) Females could have produced a child every three years or so. Males could have produced a child every night, if they could find a willing sexual partner. This theoretical difference often plays out as a practical difference. In hunter-gatherer societies, almost no woman bears more than eight children, whereas highly attractive men often sire a couple of dozen children by different women.

Before contraception, a man's reproductive success would have increased with his number of sexual partners, without limit. Every fertile woman he could seduce represented an extra potential child to carry his genes. But a woman's reproductive success reached its limit much more quickly. Conception with one partner was enough to keep her reproductively busy for the next three years. One might think that two children should be enough for each man, because that would sustain the population size. But that implies that evolution is for the good of the species, which it is not. The genes of sexually ambitious men would have quickly replaced the genes of men satisfied with just one sexual partner and two children.

Evolution pays attention to sex differences in reproductive potential because they translate into sex differences in reproductive variation. Males vary much more in the number of

children they produce, and this makes sexual reproduction a higher-risk, higher-stakes game for them. Females vary less in their quantity of children, so they care more about quality. So what do the males do with all the extra energy that females are devoting to growing eggs, being pregnant, and producing milk? They use it for reproductive competition and courtship. There is a fundamental tradeoff between courtship effort and parental effort. The more time and energy you devote to growing and raising children, the less time and energy you can devote to driving off sexual competitors and seducing sexual partners.

Jumping Ship

From the point of view of genes in any male body, the body itself is a sinking prison ship. Death comes to all bodies sooner or later. Even if a male devoted all of his energy to surviving, by storing up huge fat reserves and hiding in an armored underground compound, statistics guarantee that an accident would sooner or later kill him. This paranoid survivalist strategy is no way to spread one's genes through a population. The only deliverance for a male's genes is through an escape tube into a female body carrying a fertile egg. Genes can survive in the long term only by jumping ship into offspring. In species that reproduce sexually, the only way to make offspring is to merge one's genes with another individual's. And the only way to do that, for males, is to attract a female of the species through courtship. This is why males of most species evolve to act as if copulation is the whole point of life. For male genes, copulation is the gateway to immortality. This is why males risk their lives for copulation opportunities—and why a male praying mantis continues copulating even after a female has eaten his head.

For a female, too, the body is a sinking ship, but it has almost

everything necessary to make more bodies: eggs, womb, milk. The only thing missing is a DNA packet from a male. But there are

many willing donors. Finding a partner is usually not the problem. There are often so many willing males that the female can afford to be choosy. Quality becomes the issue. Each of the female's

offspring inherits half of its genes from whatever male she chooses. If she chooses an above-average male, her offspring get above-average genes, and are therefore more likely to survive and reproduce. It is for this reason that female mate choice

evolved.

Because females can afford to be choosy, and the benefits of sexual choice are large for them, females will typically evolve sexual preferences. As long as the males of a species invest very little in their offspring, they have no reason to refuse to copulate with any female. This is why male mate choice is rarer across species and less discriminating within each species than female mate choice. And as long as males are not sexually choosy, females do not have to bother evolving sexual ornaments. This is why sexual selection produces the sex differences we typically see in most animal species: ardent males with large sexual ornaments courting choosy females without ornaments. (This is sometimes misunderstood by critics as suggesting that males are more "active" and females more "passive." This uselessly simplistic active/passive dichotomy was not prompted by Darwin and is not accepted by modern biologists. Choosy females may be quite active in searching for good mates, comparing males, and soliciting copulations from desired males.)

If the human brain evolved through sexual selection, and followed this typical pattern, we would expect the same sex differences—not only in human behavior, but in human psychology. As far as human sexual behavior goes, the typical biological pattern outlined above seems a pretty good first approximation. Male humans generally invest more time, energy, and risk in sexual courtship, invest less in parenting, are more willing to copulate earlier in relationships with larger numbers of partners, and are less choosy about their sexual partners, at least in the short term. Female humans generally invest less in courtship and much more in parenting, are less willing to copulate early with large numbers of partners, and are more choosy. David Buss, Don Symons, Margo Wilson, Martin Daly, Laura Betzig, and many other evolutionary psychologists have gathered a mountain of data from diverse cultures documenting these sex differences and showing how they

can be explained by Darwinian sexual selection. Such studies received a great deal of media attention in the 1990s, and have destroyed the credibility of claims that human sexuality and sex differences are purely a product of culture and socialization.

However, finding the typical sex differences in humans actually makes it harder to argue coherently that sexual selection had a very significant effect on the evolution of the human mind. This is because the typical pattern of male courtship and female choice would have produced much larger sex differences in brain size, intelligence, and psychology than actually exists. Given that we now understand the origins of typical sex differences, how can the human pattern of sexually differentiated courtship result in sexually similar minds?

I do not claim to have a simple answer that explains everything about human sex differences and similarities. I can only ask for you to think through some possibilities with me. Remember, almost every theory of human mental evolution raises the same difficult issues about sex differences, because almost every theory depends on selection pressures that would have affected males and females somewhat differently.

The Sexes Share Genes

There are three factors that could have kept male human minds similar to female human minds despite strong sexual selection. The first factor is called "genetic correlation between the sexes." Males and females in every species share almost the same genes. There is a very high genetic correlation between the sexes. In humans for example, 22 pairs of our chromosomes are shared by both sexes, while only one pair, the X and Y sex chromosomes, are

sexually distinct.

The genetic correlation between the sexes inhibits the evolution of sex differences, at least in the short term. Sex differences do not spring up automatically just because sexual selection is at work. Sex differences have to evolve gradually, like everything else. Consider the example of runaway sexual selection for long tails in birds. We assumed that the long tails would be passed on only

from father to son. That might happen after many generations, but it is very unlikely to happen that way at first. It is much more likely that a mutation that increases tail length will i : passed along to both sexes. Both male and female offspring will inherit longerthan-average tails from their sexually attractive fathers. Initially, tail length will increase with equal speed in both sexes. And both male and female offspring may tend to inherit their mother's sexual preference for longer tails. So, female tail length will ride along on the genetic coattails of male tail length, and male sexual preferences will ride in tandem with female sexual preferences.

Darwin understood the genetic correlation between the sexes in a sketchy way, calling it "the law of equal transmission." In
The Descent of Man
he argued that male human intelligence and imagination evolved mainly through sexual competition, and wrote that "It is, indeed, fortunate that the law of the equal transmission of characters to both sexes has commonly prevailed throughout the whole class of mammals; otherwise it is probable that man would have become as superior in mental endowment to women, as the peacock is in ornamental plumage to the peahen." Basically, Darwin viewed the female brain as riding along on the genetic coattails of sexually selected male brains.

Genetic correlations between the sexes can be measured, and are often fairly strong. Anthropologist Alan Rogers found a very high genetic correlation between male and female height in humans, in a paper he published in 1992. This does not mean that men and women are the same average height. Nor does it just mean that tall fathers have tall daughters, and that tall mothers have tall sons. Technically, it means that a tall parent's opposite-sex offspring are almost as extreme in their height, compared to others of their sex, as their same-sex offspring are, compared to others of their own sex. Rogers saw the implications for sexual selection. If females favored taller-than-average males as sexual partners, then of course male height would increase over evolutionary time because of the sexual selection. But Rogers calculated that female height would also increase, due to the genetic correlation with male height. In fact, female height would increase

98 percent as fast as male height. As you can see, a very unequal sexual selection pressure can produce a very equal outcome.

However, these genetic correlation effects are transient. Eventually, male choosiness should decrease, and the costs of female ornamentation should increase, and these effects will break down the genetic correlation. Male choosiness would probably be eliminated first. Coming back to our long-tailed bird example, any male who rejects a short-tailed female will produce fewer offspring than a male who is less choosy. In most species, the pressures against male choosiness are very strong, causing sex differences in choosiness to evolve very fast. Sex differences in ornamentation might take a bit longer. Females with long tails will be inconvenienced by their cost, and if males do not prefer them to short-tailed females, they should evolve inhibitions against expressing the runaway male ornament. (Typically, this means that they evolve a gene expression mechanism that is sensitive to sex hormones during development, so the genes for long male tails are not turned on in female bodies.)