"Two things," the philosopher Immanuel Kant wrote, "fill the mind with ever new and increasing admiration and awe, the oftener and more steadily we reflect on them: the starry heavens above and the moral law within." And, to be sure, from Darwin to Allee, Kropotkin to Fisher, Emerson to Haldane to Wynne-Edwards, the mystery of altruism, considered the highest form of morality, was attacked from all possible directions. Where did altruism come from: Could it have been borne by the invisible hand of natural selection working directly on genes, on individuals, perhaps, on communities, on groups? Each had a hunch, and each had an answer. Still in awe, still in admiration, no one came up with an entirely convincing solution.

Then came George C. Williams. "Group-related adaptations do not, in fact, exist," he ordained; while possible in principle, genes don't evolve via...

But despite immediate criticism and however weirdly counterintuitive, it was a robust theory: As biology marched ahead it seemed to unify a plethora of phenomena. Whether animals aid their kin (ants and wolves who help their sisters breed) or nonrelatives (vampire bats who share blood, mouth to mouth, at the end of a night of prey with members of the colony who were less successful in the hunt); whether they abandon their eggs (sharks and skates and stingrays) or goslings (eagle owls and leopard-faced vultures) or sacrifice themselves for the next generation (male praying mantises serve their heads during coitus to their avaricious ladies); whether they come together as a group (Siberian steeds forming rings against predators) or aid themselves at the expense of their hosts (from the common cold bug to proliferating cancers)—all living things are acting in the interest of their true masters: a cabal of genes whose sole imperative is replication. Volition and mind and "free will" notwithstanding, evolution fashioned genes that do whatever it takes to survive.

If Williams, aided by Dawkins, helped get rid of groups, kin selection had acted as a handmaiden. Scaling the eighties and nineties into the twenty-first century, family relatedness threw massive ropes down from Mount Modern-Evolutionary-Biology for others to safely climb. Haldane's mythological drunken insight and Hamilton's resulting rule, it transpires, hold up incredibly well in the face of winds, falling rocks, and negative slopes. From the naked African mole rat, the mammalian equivalent of the termite, sometimes called the "saber-toothed sausage," which forsakes procreation in order to help its chosen monarch, to the carnivorous spadefoot toad tadpole, which can actually "taste" relatedness and therefore spits out cousins and brothers—but not strangers—that find themselves in his mouth, relatedness has proven to be a robust predictor of altruistic behavior. Even cuckoos have figured out this metric: They take advantage of other birds' familial instincts by laying their eggs in complete strangers' nests, allowing the tricked parents to shoulder the burden of parenthood. Beginning in the seventies, hundreds of biologists, ecologists, and evolutionary modelers have used Hamiltonian logic to make sense of many dramas of love and deception. With few exceptions, the general rule holds: The closer the kin, the greater the benevolence.

Two very different examples help to show just how far kin selection has captured the imagination:

Moving through soil by extending its pseudopods, most of the time the cellular slime mold, Dictyostelium discoideum, is a loner. Usually it engulfs and eats bacteria, but when times are rough and bacteria are scarce, something amazing happens: The starving amoebas secrete a chemical, cAMP, which attracts the others along a concentration gradient, until chains of tens of thousands of them merge into a mound. Soon the mound elongates into a slug that begins to crawl, as one multicellular body, across the forest floor. When it reaches a place with some heat and light it stops, and the amoebas that formed the front 20 percent of the body arrange themselves into a stalk, laying down tough cells of cellulose, just like plants, to make it nice and hardy. Then the remaining 80 percent climb up the stalk. When they reach the top they reorganize themselves into spores, forming a round glistening orb. It is this 80 percent that will stand a chance to live another day, sticking perhaps to the wings or legs of some insect, or otherwise being taken by the wind. The 20 percent that formed the stalk, on the other hand, will have sacrificed themselves altruistically for all the rest.

This is incredible, but what was discovered next is even more fascinating. In the wild most fruiting bodies form from a single clone: All the amoebas coming together to make the slug are virtually genetically identical. But when the husband-and-wife team Joan Strassman and David Queller mixed amoebas from different clones they uncovered the following: Able to recognize one another, members of one clone did their best to stick together at the backside of the slug. When the stalk was made, it was primarily they, and not the others, who shimmied up to become hopeful spores.

If amoeba can recognize and aid kin, so too, of course, can humans; this shouldn't be all that surprising. What is surprising is that studies have shown that stepchildren are not only much less likely to be invested in than biological children, but also much more likely to be abused. Surprising, that is, if your names aren't Martin Daly and Margo Wilson. This husband-and-wife team has taken kin-selection logic to its end: Just like the slime mold, they claim, and the spitting toad and the cuckoo, humans are simply following Hamilton's rule.

But if genetic relatedness was a handmaiden to the gene's-eye point of view, von Neumann games also proved a useful mountaineering partner. Soon its ropes, too, were being climbed by many a follower. The point of departure was George and Maynard Smith. Bolstered in The Selfish Gene, the concept of the ESS soon invaded the study of animal behavior. George and John, it transpired, had made an error in their paper: Retaliator, after all, was not an ESS. Since Dove did equally as well in a population of Retaliators, it could slowly drift into the population. When that happened, the true ESS would become a mixture of "Hawks" and "Bullies." George, perhaps, might not have been glad to hear about it, nor to know that his "Mouse" had once again become a "Dove." But considering that within a decade the application of game theory to evolution had revolutionized the field, perhaps he might have been assuaged nonetheless.

Once more, two illustrations from the many serve to make the point. Male dung flies, it transpires, are aptly named: Like fierce elephant seals or bucking red deer, they too defend their territory, even if in their case this is nothing but a patch of smelly excrement. The reason they do so is that females lay their eggs on the dung, and the fresher (and thus smellier) the patch, the more attractive it is to them. Having arrived earlier, males fight over the best patches; he who secures the most attractive dropping will win the right to mate with the female as she deposits her eggs. The question is: For how long should a male fly defend a patch of fresh shit before moving on to another? After all, the drier and crustier it becomes, the less chance that a female will choose to land on it. Clearly, just as in a von Neumann game, the answer depends on the actions of the other male flies. It turns out that, fashioning the minute fly a strategist, an optimal ESS can be worked out. On paper it is forty-one minutes, and incredibly, in nature it's just a few minutes away.

But if an ESS is good for flies, once again it is not too good for humans. In fact, game theory analyses of animal, and even plant and bacteria, behavior have been so successful that the modifications made specifically to fit evolutionary problems are now being retranslated back into economics. If neoclassical economic theory à la Milton Friedman assumed perfectly rational actors, it has since become clear that this is not really so: Risk aversion, status seeking, myopia, and other inbuilt cognitive biases are rampant in humans, and economic models of decision making need to take them into account. Introducing evolution-style games that assume minimal rationality, but whose dynamic depends on mutation, selection, and learning instead, has therefore become popular in economic theory. As an increasing number of theorists have found, this approach is helpful in figuring out problems like why firms don't always act to maximize their profits, or whether in a given competitive market investors should be aggressive or lazy. Darwin owed a debt to Malthus, and his followers are paying it back.

Reprinted from The Price of Altruism: George Price and the Search for the Origins of Kindness, by Oren Harman © 2010 by Oren Harman. Used with permission of the publisher, W.W. Norton & Company, Inc.

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