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tices make more sense. Throughout India it has always been the case that women more than men can "marry up, " into a higher social and economic caste, so daughters of poor people are more likely to do well than sons. In Dickemann 's analysis, dowries are merely a distorted echo of the Trivers-Willard effect in a female-exogamous species: Sons inherit the status necessary for successful breeding; daughters have to buy it. If you have no wealth to pass on, use what you have to buy your daughter a good husband. 79
Trivers and Willard predict that male favoritism in one part of society will be balanced by female favoritism elsewhere if only because it takes one of each to have a baby—the Fisher logic again.
In rodents the division seems to be based on maternal condition. In primates it seems to be based on social rank. But baboons and spider monkeys take for granted the fact that their societies are strictly stratified. Human beings do not. What happens in a modern, relatively egalitarian society?
In that uniform middle-class Eden known as California, Hrdy and her colleague Debra Judge have so far been unable to detect any wealth-related sex bias in the wills people leave when they die. Perhaps the old elite habit of preferring boys to girls has at last been vanquished by the rhetoric of equality. 8o But there is another, more sinister consequence of modern egalitarianism. In some societies the boy-preferring habit seems to have spread from elites to the society at large. China and India are the best examples of this: In China a one-child policy may have led to the deaths of 17 percent of girls. Im one Indian hospital 96 percent of women who were told they were carrying daughters aborted them, while nearly 100 percent of women carrying sons carried them to term. 81 This implies that a cheap technology allowing people to choose the gender of their children would indeed unbalance the population sex ratio.
Choosing the gender of your baby is an individual decision of no consequence to anybody else. Why, then, is the idea inherently unpopular? It is a tragedy of the commons—a collective harm that results from the rational pursuit of self-interest by individuals.
One person choosing to have only sons does nobody else any harm,
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but if everybody does it, everybody suffers. The dire predictions range from a male-dominated society in which rape, lawlessness, and a general frontier mentality would hold sway to further increases in male domination of positions of power and influence.
At the very least, sexual frustration would be the lot of many men: Laws are passed to enforce the collective interest at the expense of the individual, just as crossing over was invented to foil outlaw genes. If gender selection were cheap, a fifty-fifty sex ratio would be imposed by parliaments of people as surely as equitable meiosis was imposed by the parliament of the genes.
Chapter 5
THE PEACOCK ' S TALE
Tut, You saw her fair, none else being by, Herself poised with herself in either eye: But in that crystal scales let there be weigh'd Your lady's love against some other maid That I will show you shining at this feast, And she shall scant show well that now seems best:
—William Shakespeare, Romeo and Juliet
The Australian brush turkey builds the best compost heaps in the world. Each male constructs a layered mound of two tons of leaves, twigs, earth, and sand: The mound is just the right size and shape to heat up to the perfect temperature to cook an egg slowly into a chick. Female brush turkeys visit the males ' mounds, lay eggs in them, and depart. When the eggs hatch, the young struggle slowly to the surface of the mound, emerging ready to fend for themselves.
To paraphrase Samuel Butler ( "a hen is just an egg 's way of making another egg "), if the eggs are just the female 's way of making another brush turkey, then the mound is just the male 's way of making another brush turkey. The mound is almost as precisely a product of his genes as the egg is of hers: Unlike the female, though, the male has a residual uncertainty. How does he know that he is the father of the eggs in the mound? The answer, discovered recently by Australian scientists, is that he does not know and, in fact, is often not the father. So why does he build vast mounds to raise other males ' offspring when the whole point of sexual reproduction is for his genes to find a way into the next generation? It turns out that the female is not allowed to lay an egg in the mound until she has agreed to mate with the male; that is his price for the use of the mound. Her price is that he must then accept an egg. It is a fair bargain.
But this puts the mound in an entirely different light. From the male 's point of view the mound is not, after all, his way of making young brush turkeys. It is his way of attracting female
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brush turkeys to mate with him: Sure enough, the females select the best mounds, and therefore the best mound makers, when deciding where to lay their eggs. The males sometimes usurp one another 's mounds, so the best mound owner may actually be the best mound stealer.
Even if a mediocre mound would do, a female is wise to pick the best so that her sons inherit the mound-building, mound-stealing, and female-attracting qualities of their father. The male brush turkey 's mound is both his contribution to child rearing and a solid expression of his courtship.'
The story of the brush turkey 's mound is a story from the theory of sexual selection, an intricate and surprising collection of insights about the evolution of seduction in animals, which is the subject of this chapter. And, as will become clear in later chapters, much of human nature can be explained by sexual selection.
IS LOVE RATIONAL?
It is sometimes hard even for biologists to remember that sex is merely a genetic joint venture: The process of choosing somebody to have sex with, which used to be known as falling in love, is mysterious, cerebral, and highly selective: We do not regard any and all members of the opposite sex as adequate partners for genetic joint venture. We consciously decide whether to consider people, we fall in love despite ourselves, we entirely fail to fall in love with people who fall in love with us. It is a mightily complicated business.
It is also nonrandom. The urge to have sex is in us because we are all descended from people who had an urge to have sex with each other; those that felt no urge left behind no descendants. A woman who has sex with a man (or vice versa) is running the risk of ending up with a set of genes to partner hers in the next generation: Little wonder that she is prepared to pick those partner genes carefully. Even the most promiscuous woman does not have sex indiscriminately with anyone who comes along: The goal for every female animal is to find a mate with suf-THE PEACOCK ' S TALE
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ficient genetic quality to make a good husband, a good father, or a good sire. The goal for every male animal is often to find as many wives as possible and sometimes to find good mothers and dams, only rarely to find good wives. In 1972, Robert Trivers noticed the reason for this asymmetry, which runs right through the animal kingdom; the rare exceptions to his rule prove why it generally holds: The sex that invests most in rearing the young—by carrying a fetus for nine months in its belly, for example—is the sex that makes the least profit from an extra mating: The sex that invests the least has time to spare to seek other mates. Therefore, broadly speaking, males invest less and seek quantity of mates, while females invest more and seek quality of mates:'