The Moral Animal: Why We Are, the Way We Are: The New Science of Evolutionary Psychology (26 page)

In particular, parents should do what Trivers called "molding"
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in the guise of "teaching." He wrote: "Since teaching (as opposed to molding) is expected to be recognized by offspring as being in their own self-interest, parents would be expected to overemphasize their role as teachers in order to minimize resistance in their young."
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Trivers might view with cynicism one of Darwin's recollections of his mother: "I remember her saying 'if she did ask me to do something ... it was solely for my good.' "
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Parents have a second, more specific, advantage in trying to (partly) frustrate the genes of their children. Kin selection has ensured that the conscience looks out for siblings, generating guilt after any major neglect of them. So parents have a guilt-center to play on, and natural selection should make them good at playing on it. On the other hand, Trivers noted, natural selection should then turn around and give the children antiexploitation equipment — perhaps, for example, a pointed skepticism of parental claims about fraternal duty. Another arms race.

The result of all this is a full-fledged battle for every child's soul. Trivers has written: "The personality and conscience of the child is formed in an arena of conflict."
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Trivers sees the prevailing view of child-rearing — as a process of "enculturation," in which parents dutifully equip children with vital skills — as hopelessly naive. "One is not permitted to assume that parents who attempt to impart such virtues as responsibility, decency, honesty, trustworthiness, generosity, and self-denial are merely providing the offspring with useful information on appropriate behavior in the local culture, for all such virtues are likely to affect the amount of altruistic and egoistic behavior impinging on the parent's kin, and parent and offspring are expected to view such behavior differently." It almost seems that Trivers sees the very prevalence of the notion of "enculturation" as an unspoken conspiracy among oppressors. "The prevailing concept of socialization," he notes, "is to some extent a view one would expect adults to entertain and disseminate."
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This hints at a sense in which Darwinism, long stereotyped as a right-wing worldview, can have emanations of another sort. Seen through the new paradigm, moral and ideological discourse can look like a constant power struggle, in which the powerful often prevail
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and the weak are often exploited. "The ruling ideas of each age," wrote Karl Marx and Friedrich Engels, "have ever been the ideas of its ruling class."
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So far we have been looking at the no-frills models of kin selection and parent-offspring conflict, relying on convenient but in some cases dubious assumptions. One such assumption is that during human evolution siblings have had the same father as well as mother. To the extent that this premise is flawed — and it surely is, to some extent — then the "natural" ratio of altruism among siblings isn't two to one in favor of the self, but somewhere between two and four to one. (This amendment may ease the concerns of parents who find their offspring more mutually antagonistic than Hamilton's math seems to deem "natural.") Of course, it is possible that offspring actually (unconsciously) gauge the odds that their siblings share their father as well as their mother, and then treat them accordingly. It would be interesting, for example, to see if siblings with two homebody parents are more generous toward one another than siblings whose parents are often apart.

Another oversimplification has been the idea that r by itself — your degree of relatedness to other people — defines your genetically optimal attitude toward them. The mathematical question William Hamilton raised — Is c less than br? — has two other variables: the cost of your altruism to you (c) and the benefit to the recipient (b). Both are stated in terms of Darwinian fitness: how much the chances of producing viable, reproductively successful offspring will drop for you because of the altruism and how much they'll rise for the recipient. Both of these things, obviously, depend on what those chances were to begin with — on how much, if any, reproductive potential you and the other person have. And reproductive potential is a much-changing thing, both from one relative to the next and, for any one relative, from one decade to the next.

For example: a big, strong, smart, handsome, ambitious brother has greater likely reproductive success than a withdrawn, dull, inept brother. And this would have been especially true in the social environment of human evolution, where high-status males might have
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more than one wife — or, failing that, might be widely adulterous. In theory, parents should (consciously or unconsciously) pay attention to such differences. They should dole out investments in their various children with all the discernment of a Wall Street portfolio manager, the goal always being to maximize overall reproductive return on each increment of investment. Thus, there may be an evolutionary basis for the complaint that "Mom [or Dad] always liked you best." The Smothers Brothers comedy team made that line famous during the 1960s, and it was always the dull-witted, pie-faced Tommy making the complaint to his sharper, more dynamic brother Dick.
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The relative reproductive potential of two offspring may depend on more than the offspring themselves. It may depend on a family's social standing. For a poor family with a pretty girl and a handsome but not otherwise especially gifted boy, the daughter is more likely to produce children who begin life with material advantages; girls more often "marry up" on the socioeconomic scale than boys.
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For rich, high-status families, it's the sons who, all other things being equal, have higher reproductive potential; a man, unlike a woman, can use wealth and status to produce scores of offspring.

Are human beings programmed to execute this unsettling logic? Do parents who find themselves resource-rich or high in status unconsciously decide to lavish attention on their sons at the expense of daughters, since sons can (or could, during evolution) more efficiently convert status or material resources into offspring? Do parents who find themselves poor do the reverse? Sounds creepy, but that doesn't mean it doesn't happen.

The logic is grounded in a more general point that Robert Trivers made in 1973 in a paper coauthored with the mathematician Dan E. Willard.
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In any polygynous species, some males mate prolifically and others fail entirely to reproduce. So mothers in poor physical condition might profit (genetically) from treating daughters as more valuable assets than sons. For, assuming the mother's poor health leads — through skimpy milk, say — to frail offspring, it will bode especially ill for the sons. Malnourished males may be shut out of reproductive competition altogether, whereas a fertile female in almost any condition can usually attract a sex partner.

Some nonhuman mammals seem to comply with this logic.
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Florida pack rat mothers, if fed poorly, will force sons off the teat, even letting them starve to death, while daughters nurse freely. In other species, even the birth ratio of males to females is affected, with mothers in the most auspicious condition having mostly sons and less advantaged mothers having mostly daughters.
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In our species, somewhat polygynous through much of its evolution, wealth and status can be as important as health. Both are weapons with which men compete for women — and, in the case of status, at least,, this has been the case for millions of years. So, for parents who find themselves socially and materially advantaged, to invest more in sons than daughters would make (Darwinian) sense. This is a good example of logic that often strikes people as somehow too Machiavellian to be part of human nature. To a Darwinian, the Machiavellian coldness, if anything, adds credibility. (As Thomas Huxley said after Darwin proposed a particularly seamy hypothesis about reproduction in jellyfish: "The indecency of the process is to a certain extent in favour of its probability.")
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So far, there is evidence that the Darwinians are right, and much less evidence to the contrary. In the late 1970s, the anthropologist Mildred Dickemann, after studying nineteenth-century India and China and medieval Europe, concluded that female infanticide — the killing of newborn daughters because they are daughters — has been most intensively practiced among the upper classes.
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There is also the well-known tendency in many cultures, including Darwin's, for wealthy families to pass their largest assets down to their sons, not their daughters. (A relative of Darwin's, the early twentieth-century economist Josiah Wedgwood, noted in a study of inheritance that "It appeared to be usual among the wealthier predecessors in my sample for the sons to receive a larger share than daughters. In the case of the smaller estates, equal division is much more common.")
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Biases toward sons or daughters can assume subtler form. The anthropologists Laura Betzig and Paul Turke, working in Micronesia, found that high-status parents spend more time with sons and low-status parents with daughters.
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All these findings are consistent with the logic of Trivers and Willard: for families at the upper end of the socioeconomic scale, sons are a better investment than daughters.
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What may be the most intriguing support for the Trivers-Willard
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hypothesis is among the most recent. A study of North American families found pronounced differences in how indulgent the parents of different social classes are of boys and girls. More than half of the daughters born to low-income women were breast-fed, while fewer than half of the sons were; around 60 percent of the daughters born to affluent women were breast-fed, and nearly 90 percent of the sons. And, more dramatically, low-income women, on average, had another child within 3.5 years of the birth of a son and within 4.3 years of the birth of a daughter. In other words: in the contest over how soon to create a sibling, low-income mothers are inclined to let a daughter win; they wait longer to produce a competing target for investment. For affluent women, the opposite was true: daughters had a rival sibling within 3.2 years of birth, sons within 3.9 years.
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Presumably few if any mothers in the study knew how social status can affect the reproductive success of males and females (or, strictly speaking, how it would have done so in the environment of our evolution). This is another reminder that natural selection tends to work underground, by shaping human feelings, not by making humans conscious of its logic.
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Though these studies all focused on parental investment, the same logic would apply to sibling investment. If you're poor, you should, in theory, direct more altruism toward a sister than a brother, and if you're rich, vice versa. It is certainly true that, in Darwin's well-to-do family, his sisters spent much time worrying about and catering to their brothers. But that trend may have been just as pronounced among the lower classes back when the subservience of women was a social ideal (a reminder that culture can push our behavior against the grain of Darwinian logic).

Besides, extraordinary female helpfulness has other Darwinian explanations. Reproductive potential changes over the life cycle, and changes differently for males and females. In his 1964 paper, Hamilton noted abstractly that, "the behaviour of a post-reproductive animal may be expected to be entirely altruistic."
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After all, once the vehicle that genes inhabit can't transmit them to the next generation, they are well advised to direct all energies toward vehicles that can. Since only women spend much of their lives in post-reproductive mode, the implication is that older women, much mori
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than older men, will shower attention on kin. And they do. The single aunt who devotes her life to relatives is a much more common sight than the single uncle who does the same. Darwin's sister Susan and his brother Erasmus were both middle-aged and unmarried when their sister Marianne died, but it was Susan who adopted her children.
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Even for a male, reproductive potential changes somewhat over time. In fact, it changes every year for everyone. A fifty-year-old of either sex has, on average, many fewer potential offspring in his or her future than he or she did at thirty — at which point, in turn, the potential was less than it had been at fifteen. On the other hand, the average fifteen-year-old has more future offspring than the average one-year-old, since the one-year-old may yet die before adolescence — a fairly common thing during much of human evolution.

Here lies another oversimplification in the no-frills model of kin selection. Since reproductive potential is embedded in both the cost and benefit sides of the altruism equation, the age of both the giver and the recipient help determine whether the altruism will tend to raise inclusive fitness, and thus be favored by natural selection. In other words, how warm and generous we feel toward kin depends, theoretically, both on our age and on the kin's age. There should, for example, be continuous change, throughout a child's life, in how dear that life seems to parents.
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Specifically, parental devotion should grow until around early adolescence, when reproductive potential peaks, and then begin to drop. Just as a horse breeder is more disappointed by the death of a thoroughbred the day before its first race than the day after its birth, a parent should be more heartbroken by the death of an adolescent than by the death of an infant. Both the adolescent and the mature racehorse are assets on the brink of bringing rewards, and in both cases it will take much time and effort, starting from scratch, to get another asset to that point. (This isn't to say a parent will never feel more tender or protective toward an infant than toward an adolescent. If, say, a marauding band approaches, a mother's natural impulse
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will be to grab the infant before fleeing, thus leaving the adolescent to fend for itself; but this impulse exists because adolescents can fend for themselves, not because they're less precious than infants.)