Families - Social Cement

The Moral Animal - Whe We Are The Way We Are: The New Science of Evolutionary Psychology - Robert Wright 1995

Social Cement

[With the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. It may well be asked how is it possible to reconcile this case with the theory of natural selection?

The Origin of Species (1859)

[Yesterday] Doddy [Darwin's son William] was generous enough to give Anny the last mouthful of his gingerbread & today ... he again put his last crumb on the sofa for Anny to run to & then cried in rather a vain-glorious tone "oh kind Doddy" "kind Doddy."

Observations of Darwin's children (1842)1

We all like to think of ourselves as selfless. And on occasion we are. But we are pigs compared to the social insects. Bees die for their fellow bees, disemboweling themselves upon stinging an intruder. Some ants, also in defense of the colony, detonate themselves. Other ants spend their lives as doors, keeping out insects that lack security clearance, or as food sacks, hanging bloated from the ceiling in case of scarcity.2 These pieces of furniture have no offspring.

Darwin spent more than a decade wondering how natural selection could have produced whole castes of ants that create no descendants. Meanwhile, he was creating plenty of descendants himself. The problem of insect sterility had gotten his attention by the time his {155} fourth child, Henrietta, was born in late 1843, and he still had not solved it by the birth of his tenth and last child, Charles, in 1856. For all those years, he kept the theory of natural selection secret, and one reason may be the seemingly blatant contradiction of it by ants. The paradox seemed "insuperable, and actually fatal to my whole theory."3

Darwin probably didn't suspect, as he pondered the insect puzzle, that the solution to it could explain, as well, the texture of his growing family's everyday life: why his children showed affection for one another, why they sometimes fought; why he felt compelled to teach them the virtues of kindness, why they sometimes resisted; even why he and Emma grieved more deeply the loss of one of their children than another. Understanding self-sacrifice among insects would unlock the dynamics of family life among mammals, including people.

Though Darwin finally conceived, at least vaguely, the correct explanation of insect sterility, and suspected that it might have relevance to human behavior, he came nowhere near seeing the breadth and diversity of the relevance. Neither did anyone else until a century later.

One reason for this may be that Darwin's explanation, as he phrased it, was hard to grasp. In The Origin of Species he wrote that the paradox of evolved sterility "is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Thus, a well-flavoured vegetable is cooked, and the individual is destroyed; but the horticulturist sows seeds of the same stock, and confidently expects to get nearly the same variety; breeders of cattle wish the flesh and fat to be well marbled together; the animal has been slaughtered, but the breeder goes with confidence to the same family."4

However strange it may seem to bring plant and animal breeders into the picture, this made perfect sense after 1963, when a young British biologist named William D. Hamilton sketched out the theory of kin selection.5 Hamilton's theory is an articulation and extension of Darwin's insight in the language of genetics, a language that didn't exist in Darwin's day.

The term kin selection itself suggests a link with Darwin's assertion {156} that "selection may be applied to the family," and not just to the individual organism. But this suggestion, while true, is misleading. The beauty of Hamilton's theory is that it sees selection as taking place not so much at the level of the individual or the family, but, in an important sense, at the level of the gene. Hamilton was the first to clearly sound this central theme of the new Darwinian paradigm: looking at survival from the gene's point of view.

Consider a young ground squirrel that has not yet produced any offspring and that, upon sighting a predator, gets up on its hind legs and delivers a loud alarm call, which may attract the predator's attention and bring sudden death. If you look at natural selection the way almost all biologists looked at it through the mid-twentieth century — a process concerned with the survival and reproduction of animals, and of their offspring — this warning call doesn't make sense. If the ground squirrel giving it has no offspring to save, then the warning call is evolutionary suicide. Right? This is the question that was momentously answered in the negative by Hamilton.

In the Hamiltonian view, attention shifts from the ground squirrel that is sounding the alarm to the gene (or, in real life, the series of genes) responsible for the alarm. After all, ground squirrels don't live forever, and neither do any other animals. The only potentially immortal organic entity is a gene (or, strictly speaking, the pattern of information encoded in the gene, since the physical gene itself will pass away after conveying the pattern through replication). So, in an evolutionary time frame, over hundreds or thousands or millions of generations, the question isn't how individual animals fare; we all know the finally grim answer to that one. The question is how individual genes fare. Some will pass away and some will thrive, and which do which is a matter of consequence. How will a "suicidal warning call" gene fare?

The somewhat surprising answer, which lay at the core of Hamilton's theory, is: quite well, under the right circumstances. The reason is that the ground squirrel containing the gene may have some nearby relatives who will be saved by the alarm call, and some of those relatives probably carry the same gene. Half of all brothers and sisters, for example, can be assumed to possess the gene (unless they're half-siblings, in which case the fraction is a still nontrivial one-fourth). {157}

If the warning call saves the lives of four full siblings that would otherwise die, two of which carry the gene responsible for it, then the gene has done well for itself, even if the sentry containing it pays the ultimate sacrifice. This superficially selfless gene will do much better over the ages than a superficially selfish gene that induced its carrier to scurry to safety while four siblings — and two copies of the gene, on average — perished.* The same is true if the gene saves only one full sibling, while giving the sentry a one-in-four chance of dying. Over the long run, there will be two genes saved for every gene lost.


There is nothing mystical going on here. Genes don't magically sense the presence of copies of themselves in other organisms and try to save them. Genes aren't clairvoyant, or even conscious; they don't "try" to do anything. But should a gene appear that happens to make its vehicle behave in ways that help the survival or reproductive prospects of other vehicles likely to contain a copy of that gene, then the gene may thrive, even if prospects for its vehicle are lowered in the process. This is kin selection.

This logic could apply, as in this case, to a gene that inclines a mammal to produce a warning call when it sees a threat to its home burrow, where relatives reside. The logic could also apply to a gene that leads an insect to be sterile, so long as the insect spends its life helping fertile relatives (who contain the gene in "unexpressed" form) to survive or reproduce. And the logic could apply to genes inclining human beings to sense early on who their siblings are and thereafter share food with them, give guidance to them, defend them, and so on — genes, in other words, leading to sympathy, empathy, compassion: genes for love.

A failure to appreciate familial love had helped keep the principle of kin selection from clear view before Hamilton's day. In 1955, in {158} a popular article, the British biologist J.B.S. Haldane had noted that a gene inclining you to jump in a river and save a drowning child, taking a one-in-ten chance of dying, could flourish so long as the child were your offspring or your brother or sister; the gene could even spread, at a slower rate, if that child were your first cousin, since first cousins share, on average, one eighth of your genes. But rather than sustain this train of thought, he cut it short by observing that in emergencies people don't have time to make mathematical calculations; and surely, he said, our Paleolithic ancestors hadn't run around calculating their degree of relatedness to each other. So Haldane concluded that genes for heroism would spread only "in rather small populations where most of the children were fairly near relatives of the man who risked his life."6 In other words: an indiscriminate heroism, reflecting the average degree of relatedness to people in the general vicinity, could evolve if that average were fairly high.

For all Haldane's insight in looking at things from the gene's, rather than the individual's, point of view, his failure to follow this logic to its end is odd, to say the least. It's as if he thought natural selection realizes its calculations by having organisms consciously repeat them, rather than by filling organisms with feelings that, in their fine contours, are proxies for calculation. Hadn't Haldane noticed that people tend to have the warmest feelings for the people who share the largest fraction of their genes? And that people are more inclined to risk their lives for the people they feel warmly toward? Why should it matter that Paleolithic men weren't math whizzes? They were animals; they had feelings.

Technically speaking, Haldane was right insofar as he went. Within a small, closely related population, an indiscriminate altruism could indeed evolve. And that's true even though some of the altruism would get spent on people who weren't relatives. After all, even if you channel your altruism precisely toward siblings, some of it is wasted, in evolutionary terms, since siblings don't share all your genes, and any given sibling may not carry the gene responsible for the altruism. What matters, in both cases, is that the altruism gene tends to improve prospects for vehicles that will tend to carry copies of itself; what matters is that the gene does more good than harm, in the long run, to its own proliferation. Behavior always takes place {159} amid uncertainty, and all natural selection can do is play the odds. In the Haldane scenario the way to play the odds is to instill a mild and generalized altruism, the exact strength depending on the average extent of kinship with people regularly in the vicinity. This is conceivable.

But as Hamilton noted in 1964, natural selection will, given the opportunity, improve the odds by minimizing uncertainty. Any genes that sharpen the precision with which altruism is channeled will thrive. A gene that leads a chimpanzee to give two ounces of meat to a sibling will eventually prevail over a gene that leads it to give an ounce to a sibling and an ounce to an unrelated chimp. So unless identifying kin is very hard, evolution should produce a strong and well-targeted strain of benevolence, not a weak and diffuse strain. And that is what has happened. It has happened, at least to some extent, with ground squirrels, which are more likely to deliver warning calls in the presence of close kin.7 It has happened, to some extent, with chimpanzees and other nonhuman primates, which often have uniquely supportive sibling relationships. And it has happened, to a great extent, with us.

Maybe the world would be a better place if it hadn't. Brotherly love in the literal sense comes at the expense of brotherly love in the biblical sense; the more precisely we bestow unconditional kindness on relatives, the less of it is left over for others. (This, some believe, is what kept Haldane, a Marxist, from facing the truth.) But, for better or worse, the literal kind of brotherly love is the kind we have.

Many social insects recognize their kin with the help of chemical signals called pheromones. It is less clear how humans and other mammals figure out (consciously or unconsciously) who their kin are. Surely seeing our mother feed and care for a child day after day is one conspicuous cue. We may also, by observing our mother's social affiliations, develop a sense for, say, who her sister is, and hence who her sister's offspring are. Besides, since the advent of language, mothers have been able to tell us who's who — instruction it is in their genetic interest to give and in our genetic interest to heed. (That is to say, genes inclining the mother to help children identify kin would thrive, as would genes inclining children to pay attention.) It's hard to say what other kin-recognition mechanisms, if any, are {160} at work, since experiments that might settle the question involve unethical things like removing children from families.8

What's clear is that mechanisms exist. Anyone with siblings — anyone in any culture — is familiar with the empathy for a sibling in great need, the sense of fulfillment at giving aid, the guilt at not giving it. Anyone who has endured a sibling's death is familiar with grief. These people know what love is, and they have kin selection to thank for it.

That goes double for males, who, in the absence of kin selection, might never have felt deep love at all. Back before our species became high in male parental investment, there was no reason for males to be intensely altruistic toward offspring. That sort of affection was the exclusive province of females, in part because only they could be sure who their offspring were. But males could be pretty sure who their brothers and sisters were, so love crept into their psyches via kin selection. Had males not thus acquired the capacity for sibling love, they might not have been so readily steered toward high male parental investment, and the even deeper love it brings. Evolution can only work with the raw materials that happen to be lying around; if love for certain kinds of children — siblings — hadn't been part of males' minds several million years ago, the path to loving their own children — the path to high MPI — might have been too tortuous.


With Hamilton's theory in hand, it's easier to appreciate the connection Darwin saw between a cow that has "well marbled" beef, and gets slaughtered and eaten, and an ant that works hard all its life without issue. The cow gene responsible for the good marbling, to be sure, has done nothing for its vehicle, which is now slaughtered, and may do nothing for the direct genetic legacy of its vehicle; dead cows can't have more offspring. But the gene will still do much for the indirect genetic legacy of its vehicle, for by producing the marbling, it prompts a farmer to feed and breed the vehicle's close relatives, some of which contain copies of the gene. So too with the sterile ant. The ant has no direct legacy, but the genes responsible for this fact do just fine, thank you, so long as the time and energy {161} that would have been devoted to reproduction are profitably spent helping close relatives be prolific. Though the gene for sterility lies dormant in these relatives, it is there, and passes to the next generation, where it again produces gobs of sterile altruists devoted to its transmission. This is the exact sense in which worker bees and tasty cattle are alike: some genes, by impeding their transmission through one conduit, lubricate their transmission through others, and the net result is more transmission.

That Darwin, working with no knowledge of genes, with no sound understanding of the nature of heredity, should sense this parallel a century before Hamilton is one of the higher tributes to the care and precision of his thought.

Still, let there be no doubt about the superiority of Hamilton's version of kin selection to Darwin's. It is accurate enough to say, as Darwin did, that sometimes (as with insect sterility) natural selection operates on the family and sometimes on the individual organism. But why not keep things simple? Why not just say that in both of these cases, the ultimate unit of selection is the gene? Why not make a single brief statement that encompasses all forms of natural selection? Namely: those genes that are conducive to the survival and reproduction of copies of themselves are the genes that win. They may do this straightforwardly, by prompting their vehicle to survive, beget offspring, and equip the offspring for survival and reproduction. Or they may do this circuitously — by, say, prompting their vehicle to labor tirelessly, sterilely, and "selflessly," so that a queen ant can have lots of offspring containing them. However the genes get the job done, it is selfish from their point of view, even if it seems altruistic at the level of the organism. Hence the title of Richard Dawkins's book, The Selfish Gene. (The title has caught flack from people who note that genes don't have intentions, and so can't be "selfish." True, of course, but the phrase wasn't meant literally.)

Naturally, the level of the organism is of primary concern to human beings; human beings are organisms. But it's of secondary importance to natural selection. If there is a sense in which natural selection "cares" about anything — and there is, metaphorically — that thing isn't us; it's the information in our sex cells, our eggs and our sperm. Of course, natural selection "wants" us to behave in certain {162} ways. But, so long as we comply, it doesn't care whether we are made happy or sad in the process, whether we get physically mangled, even whether we die. The only thing natural selection ultimately "wants" to keep in good shape is the information in our genes, and it will countenance any suffering on our part that serves this purpose.

This was the philosophical import of the simple point Hamilton made abstractly, skeletally, in a 1963 letter to the editors of the journal The American Naturalist. He imagined a gene G that causes an altruistic behavior and noted: "Despite the principle of 'survival of the fittest' the ultimate criterion which determines whether G will spread is not whether the behavior is to the benefit of the behaver but whether it is to the benefit of the gene G; and this will be the case if the average net result of the behavior is to add to the gene-pool a handful of genes containing G in higher concentration than does the gene-pool itself."9

Hamilton gave flesh to this observation the next year with his paper "The Genetical Evolution of Social Behaviour" in The Journal of Theoretical Biology. The paper, after going underappreciated for years, has become one of the most widely cited works in the history of Darwinian thought, and has revolutionized the mathematics of evolutionary biology. Before the theory of kin selection, it was common to talk as if the final arbiter in evolution were "fitness," whose ultimate manifestation, it seemed, was the sum total of the organism's direct biological legacy. Genes that made an organism fitter — that maximized the number of offspring, grand-offspring, and so on — would be the genes that flourished. Now the final arbiter of evolution is thought of as "inclusive fitness," which takes into account also the genes' indirect legacy, realized via siblings, cousins, and so on. Hamilton wrote in 1964: "Here then we have discovered a quantity, inclusive fitness, which under the conditions of the model tends to maximize in much the same way that fitness tends to maximize in the simpler classical model."

Hamiltonian math contains a potent symbol — r — introduced earlier by the biologist Sewall Wright but now given new consequence; r represents the degree of relatedness among organisms. Among full siblings, r is V2, among half-siblings, nieces, nephews, aunts, and uncles, it is lA, and among first cousins it is Vs. The new math says {163} that genes for sacrificial behavior will thrive so long as the cost to the altruist (in terms of impact on future reproductive success) is less than the benefit to the recipient (ditto) times the degree of relatedness between the two. That is, so long as c is less than br.

When Hamilton introduced the theory of kin selection, he used as his example the very group of organisms that had perplexed Darwin. Like Darwin, he had been struck by the extraordinary self-sacrifice among many insects of the order Hymenoptera, notably the highly social ants, bees, and wasps. Why is this intensity of altruism, and its attendant social cohesion, found in so few other parts of the insect world? There may be several evolutionary reasons, but Hamilton put his finger on what seems a central one. He noted that, thanks to a bizarre form of reproduction, these species feature an unusually large r. Sister ants share 3/4 of their genes by common descent, not just 1/2. So altruism of extraordinary magnitude is justified in the eyes of natural selection.

When r is even larger than 3/4, the evolutionary argument for altruism, and social solidarity, grows even stronger. Consider the cellular slime mold, which is so tightly interwoven that it has inspired reasoned debate as to whether it is best thought of as a society of cells or a single organism. Because slime-mold cells reproduce asexually, the r among them is 1; they are all identical twins. From the point of view of the gene, then, there is no difference between the fate of its own cell and the fate of a nearby cell. It's not surprising that many slime-mold cells fail to reproduce, and devote themselves instead to buffering fertile fellow cells from the elements. Their neighbors' welfare, in evolutionary terms, is identical to their own. That's altruism.

So too with human beings — not groups of human beings, but the groups of cells that are human beings. At some point hundreds of millions of years ago, multicellular life arose. Societies of cells became so highly integrated as to qualify for the title "organism," and these organisms eventually begat us. But as the cellular slime mold attests, the line between society and organism is unclear. It is fair, technically speaking, to consider even so coherent an organism as a human being a tight-knit community of single-celled organisms. These cells exhibit a kind of cooperation and self-sacrifice that makes even the {164} machine-like efficiency of an insect colony look ragged by comparison. Almost all of the cells in the human body are sterile. Only the sex cells — our "queen bees" — get to make copies of themselves for posterity. That the zillions of sterile cells act as if they were perfectly content with this arrangement is doubtless grounded in the fact that the r between them and the sex cells is 1; genes in sterile cells are transmitted to future generations as assuredly via sperm or egg as they would be if their particular cellular vehicles were doing the transmitting. Again: when r is 1, altruism is ultimate.


The reverse side of this coin is that when r isn't 1, altruism isn't ultimate. Even pure sibling love — brotherly love — isn't total love. J.B.S. Haldane is said to have remarked once that he would never give his life for a brother — but, rather, for "two brothers or eight cousins." Presumably he was joking — parodying, perhaps, what he wrongly considered the overly fine extension of Darwinian logic. But his joke captures a basic truth. To define the degree of commitment to any relative is to define the degree of indifference and, potentially, antagonism; the cup of common interest between siblings is half-empty as well as half-full. While it makes genetic sense to help a brother or sister, even at great expense, that expense is not unlimited.

Thus, on the one hand, no modern Darwinian would expect a child to monopolize the food supply while a brother or sister grew weak from hunger. But neither should we expect that, given two siblings and one sandwich, the question of its allotment will be amicably resolved. It may not be hard to teach children to share with brothers and sisters (at least in some circumstances), but it is hard to teach them to share equally, for this runs against their genetic interest. That, at any rate, is what natural selection implies. We can leave it for veteran parents to say whether the prediction is borne out.

The divergence of genetic interests between siblings creates an exasperating, if sometimes charming, paradox. They fiercely compete for the affection and attention of their parents, with all the resources that can bring, and in the process display jealousy so petty that it's hard to credit them with love; but let one of them become truly {165} needy, or seem genuinely endangered, and love will surface. Darwin saw one such shift in attitude on the part of his son Willy, then nearly five years old, toward younger sister Annie. "Whenever she hurts herself when we are present Willy appears not to mind, & sometimes makes a great noise as if to distract our attention," Darwin wrote. But one day Annie hurt herself with no adults in view, so Willy couldn't assume that any real danger was being addressed. Then his reaction "was quite different. He first attempted to comfort her very nicely & then said he would call Bessy & she not being in sight his fortitude gave way & he began to cry also. "10 Darwin didn't explain this, or any instances of sibling love, in terms of kin, or what he called "family," selection; he seems never to have seen the connection between insect self-sacrifice and mammalian affection.11

The biologist who first emphasized the partial emptiness of the cup of common genetic interest is Robert Trivers. He has noted, in particular, that a child's genetic interest diverges not only from a brother's or sister's, but from a parent's. Each child should, in theory, see itself as twice as valuable as its sibling, while the parent, being equally related to the two, values them equally. Hence another Darwinian prediction: not only will siblings have to be taught to share equally; parents will, in fact, try to teach them.

In 1974, Trivers dissected parent-offspring conflict in a paper by that name. By way of illustration, he discussed the contentious mammalian issue of when a suckling should get off the teat. A caribou calf, he observed, will continue to suckle long after milk has ceased to be essential to its survival, even though this prevents the mother from conceiving another calf that will share some of its genes. After all: "the calf is completely related to himself but only partially related to his future siblings...."12 The time will come when the nutritional rewards from suckling are so marginal that genetic interest favors another calf over milk. But the mother, valuing (implicitly) the two offspring equally, reaches that point sooner. So the theory of natural selection, stated in terms of inclusive fitness, implies that conflict over weaning will be a regular part of mammalian life — as it seems to be. The conflict can last for several weeks and become pretty wild, as infants shriek for milk and even strike their mother. Veteran baboon {166} watchers know that a good way to find a baboon troop is to listen each morning for the sound of mother-offspring strife.13

In the battle over resources, expect children to use any tools at their disposal, including dishonesty. The dishonesty may be crude and directed at other siblings. ("Willy sometimes tries a little ruse to prevent Annie wishing to have his apple... .'Yours is larger than mine Annie.' ") But the ruse may be more subtle, and directed to a larger audience, including parents. One good way to short-circuit parental demands for greater sacrifice is to exaggerate — or, shall we say, selectively highlight — sacrifices already made. An example appears at the head of this chapter: Willy, then two years old, and nicknamed Doddy, had given his younger sister his last bit of gingerbread and then exclaimed, for all to hear, "Oh kind Doddy, kind Doddy."14 Many parents are familiar with this sort of conspicuous nonconsumption.

Another ploy children use to extract resources from parents is to embellish their needs. Emma Darwin recorded three-year-old son Leonard's actions after "he scraped 2 little bits of skin off his wrist": "He thought Papa did not pity him enough & nodded emphatically at him. 'The skin's come off — & its lost — & the bleed's coming out.' " A year later Leonard was heard to say, "Papa, I have coughed awfully — many times awfully — five awfullys — and more, too — so mayn't I have some black stuff [licorice]?"15

To further bolster an image of entitlement, youngsters may stress the cruelty and injustice being inflicted by parents. At peak intensity, this emphasis is known as a temper tantrum — part of young life not just in our species, but also among chimpanzees, baboons, and other primates. Many a young outraged chimp has been known, as one primatologist put it half a century ago, to "glance furtively at its mother or the caretaker as if to discover whether its action was attracting attention."16

Fortunately for young primates, parents are ripe for exploitation. Attention to a child's crying and complaining is in the interest of the parent's genes, since cries and complaints may signal real needs felt by a vehicle containing copies of them. In other words: parents love their children and can be blinded by that love. {167}

Still, the idea of a temper tantrum as manipulation won't strike most parents as a revolutionary insight, and that's evidence that they aren't entirely blind. Though natural selection made parents open to manipulation in the first place, it should, in theory, have equipped them thereafter with antimanipulation devices, such as a discerning suspicion of childhood whining. But once this discernment exists, natural selection should equip the children with counterdiscernment technology in the form of more heartfelt whining. The arms race goes on forever.

As Trivers stressed in his 1974 paper, the gene's-eye view implies that parents are themselves dishonestly manipulative. They want — or, at least, their genes "want" them — to extract from the child more kin-directed altruism and sacrifice, and thus to instill in the child more love, than is in the child's genetic interest. This is true not just of sibling love, but of love for uncles, aunts, and cousins, all of whom (on average) have twice as many of the parent's genes as of the child's. Hence the general dearth of disputes in which a parent demands that a child be less considerate of the parent's siblings, nieces, and nephews.

Children are biologically vulnerable to a parent's propaganda campaign, just as the parent is vulnerable to the child's. The reason is that it so often makes Darwinian sense to do what parents say. Although the genetic interests of parent and child diverge, there is a 50 percent overlap, so no one has a stronger genetic incentive than a parent to fill a child's head with useful facts and adages. Thus there is no one to whom the child should pay more attention. A child's genes should "want" the child to tap into the uniquely devoted databanks housed in its parents.

And plainly, the genes get their way. When young, we are filled with awe and credulity in the presence of our parents. One of Darwin's daughters recalled that "Whatever he said was absolute truth and law to us." Surely she exaggerates. (When Darwin found five-year-old Leonard jumping on the sofa and told him that was against the rules, Leonard replied, "Well then I advise you to go out of the room.")17 Still, young children do have a basic, if not entire, trust in their parents, and parents should, in theory, abuse it.

In particular, parents should do what Trivers called "molding" {168} 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."18 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.' "19

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."20

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."21

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 {169} 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."22


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 {170} 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.23

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.24 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.25 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. {171} 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.26

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.")27 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.28 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.")29 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.30 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.31

What may be the most intriguing support for the Trivers-Willard {172} 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.32 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.33

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."34 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 {173} 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.35


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.36

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 {174} 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.)

As predicted, parents do grieve more over the death of an adolescent than of a three-month-old — or, also in keeping with theory, of a forty-year-old. It is tempting to dismiss such results: of course we regret a young man's death more than an older man's; it's obviously tragic to die with so much of life unlived. To which Darwinians reply: Yes, but remember — the very "obviousness" of the pattern may be a product of the same genes that, we propose, created it. The way natural selection has worked its will is to make some things seem "obvious" and "right" and "desirable" and others "absurd" and "wrong" and "abhorrent." We should probe our commonsense reactions to evolutionary theories carefully before concluding that the common sense itself isn't a cognitive distortion created by evolution.

In this case, we should ask: If an adolescent boy's vast unlived life is what makes his death seem so sad, why doesn't the death of an infant seem even sadder? One answer is that we've had more time to get to know the adolescent and can thus see the unlived life more clearly. But what a coincidence that the countervailing changes in these two quantities — the growing intimacy with a person over time and the shrinking size of that person's unlived life — happen to reach some sort of maximum combined grief value right around adolescence, when that person's reproductive potential is highest. Why doesn't the peak come, say, at age twenty-five, when the contours of the unlived life are really clear? Or at age five, when there's so much unlived life?

The evidence so far is that grief does comply exquisitely with Darwinian expectations. In a 1989 Canadian study, adults were asked to imagine the death of children of various ages and estimate which deaths would create the greatest sense of loss in a parent. The results, plotted on a graph, show grief growing until just before adolescence and then beginning to drop. When this curve was compared with a curve showing changes in reproductive potential over the life cycle (a pattern calculated from Canadian demographic data), the correlation was fairly strong. But much stronger — nearly perfect, in {175} fact — was the correlation between the grief curve of these modern Canadians and the reproductive-potential curve of a hunter-gatherer people, the !Kung of Africa. In other words, the pattern of changing grief was almost exactly what a Darwinian would predict, given demographic realities in the ancestral environment.37

In theory, and in fact, the dearness of parents to children also changes over time. In the pitiless eyes of natural selection, the utility of our parents to us declines, after a certain point, even faster than ours to them. As we pass through adolescence, they are less and less critical databanks, providers, and protectors. And as they pass through middle age, they are less and less likely to further promulgate our genes. By the time they are old and infirm, we have little if any genetic use for them. Even as we attend to their needs (or pay someone else to), we may feel traces of impatience and resentment. Our parents, in the end, are as dependent on us as we once were on them, yet we don't look after their needs with quite the same gusto they brought to ours.

The ever-shifting but almost perennially uneven balance of affection and obligation between parent and child is one of life's deepest and most bittersweet experiences. And it illustrates how imprecise the genes can be in turning on and off our emotional spigots. Though there seems to be no good Darwinian reason to spend time and energy on an old, dying father, few of us would, or could, turn our backs. The stubborn core of familial love persists beyond its evolutionary usefulness. Most of us, presumably, are glad for this crudeness of genetic control — although, of course, there's no way of knowing what our opinion would be if the controls were more precise.


Darwin had many occasions to grieve, including the deaths of three of his ten children and of his father. His behavior generally matches theory.

The death of the Darwins' third child, Mary Eleanor, came only three weeks after her birth in 1842. Charles and Emma were undeniably saddened, and the funeral was hard on Charles, but there are no signs of overwhelming or lasting grief. Emma wrote that, "Our {176} sorrow is nothing to what it would have been if she had lived longer and suffered more," assuring her sister-in-law that, with two other children to distract her and Charles, "you need not fear that our sorrow will last long."38

The death of the last Darwin child, Charles Waring, should also, in theory, have been a glancing blow. He was young — a year and a half — and was retarded. One of the most straightforward Darwinian predictions is that parents will care relatively little for children who are so defective as to have negligible reproductive value. (In many preindustrial societies, infants with obvious defects have been routinely killed, and even in industrial societies, handicapped children are especially prone to abuse.)39 Darwin wrote a short memorial to his dead son, but it was, in places, clinically detached ("He often made strange grimaces & shivered, when excited ...") and was nearly devoid of anguish.40 One of the Darwin daughters later said of the baby: "Both my father and mother were infinitely tender towards him, but, when he died in the summer of 1858, after their first sorrow they could only feel thankful."41

Nor should the death of Darwin's father in 1848 have been devastating. Charles was by now easily self-sufficient, and his father, at age eighty-two, had spent his reproductive potential. Darwin did show signs of deep grief in the days after the death, and of course there's no way of being sure he didn't keep suffering for months. But in his letters he never got more effusive than to note that "no one, who did not know him, would believe that a man above 83 years old [sic], could have retained so tender & affectionate a disposition, with all his sagacity unclouded to the last." He wrote three months after the death, "[W]hen last I saw him he was very comfortable & his expression which I have now in my mind's eye serene & cheerful."42

Clearly distinct from all three of these cases was the de?.th of the Darwins' daughter Annie in 1851, after a periodic illness that had begun the year before. She was ten years old, her reproductive potential just a few years from its peak.

In the days leading up to her death, there is an anguished and poignant exchange of letters between Charles, who had traveled with her to a doctor, and Emma. A few days after the death, Darwin {177} composed a memorial to Annie that is strikingly different in tone from the later memorial to Charles Waring. "Her joyousness and animal spirits radiated from her whole countenance, and rendered every movement elastic and full of life and vigour. It was delightful and cheerful to behold her. Her dear face now rises before me, as she used sometimes to come running downstairs with a stolen pinch of snuff for me, her whole form radiant with the pleasure of giving pleasure. ... In the last short illness, her conduct in simple truth was angelic. She never once complained; never became fretful; was ever considerate of others, and was thankful in the most gentle, pathetic manner for everything done for her... . When I gave her some water, she said, 'I quite thank you;' and these, I believe, were the last precious words ever addressed by her dear lips to me." He wrote in closing, "We have lost the joy of the household, and the solace of our old age. She must have known how we loved her. Oh, that she could now know how deeply, how tenderly, we do still and shall ever love her dear joyous face! Blessings on her!"43

It is possible to inject this analysis of Darwin's grief with (believe it or not) a bit more cynicism. Annie, it seems, was the Darwins' favorite child. She was bright and talented ("a second Mozart," Darwin once said) — assets that would have raised her value on the marriage market, and hence her reproductive potential. And she was an exemplary child, a model of generosity, morals, and manners.44 Or, as Trivers might put it: Emma and Charles had successfully conned her into pursuing their inclusive fitness at the expense of hers. Perhaps an analysis of "favorite children" would confirm that they tend to possess these sorts of valuable attributes — valuable from the perspective of the parents' genes, which may or may not imply value from the perspective of the child's.

Only months after his father's death, Darwin had declared his grieving at an end, referring in a letter to "my dear Father about whom it is now to me the sweetest pleasure to think."45 In the case of Annie, no such point was reached for either Emma or Charles. Another of their daughters, Henrietta, would later write that "it may almost be said that my mother never really recovered from this grief. She very rarely spoke of Annie, but when she did the sense of loss was always there unhealed. My father could not bear to reopen his {178} sorrow, and he never, to my knowledge, spoke of her." Twenty-five years after Annie's death, he wrote in his autobiography that thinking of her still brought tears to his eyes. Her death, he wrote, had been the "only one very severe grief" the family had suffered.46

In 1881, after Darwin's brother Erasmus had died, and less than a year, in fact, before Darwin's own death, he was moved to remark, in a letter to his friend Joseph Hooker, on the difference between "the death of the old and young." He wrote, "Death in the latter case, when there is a bright future ahead, causes grief never to be wholly obliterated."47 {179}