Charles Darwin regarded the problem of altruism as a potentially fatal challenge to his theory of natural selection. After all, if life were such a cruel “struggle for existence,” then how could a selfless individual ever live long enough to reproduce? Why would natural selection favor a behavior that made us less likely to survive? And yet, as Darwin knew, altruism is everywhere, a stubborn anomaly of nature. For a century after Darwin, altruism remained a paradox. The first glimmers of a solution arrived in the nineteen-fifties. According to legend, the biologist J. B. S. Haldane was asked how far he would go to save the life of another person. Haldane thought for a moment, and then started scribbling numbers on the back of a napkin. “I would jump into a river to save two brothers, but not one,” Haldane said. “Or to save eight cousins but not seven.” His answer summarized a powerful scientific idea. Because individuals share much of their genome with close relatives, a trait will also persist if it leads to the survival of their kin.
Haldane never expanded his napkin calculations into a formal mathematical theory. That task fell to William Hamilton. In 1964, he submitted a pair of papers to the Journal of Theoretical Biology. The papers hinged on one simple equation: rB > C. Genes for altruism could evolve if the benefit (B) of an action exceeded the cost (C) to the individual once relatedness (r) was taken into account. Hamilton referred to his model as “inclusive fitness theory.” At first, Hamilton’s concept of inclusive fitness was entirely ignored. Many biologists were turned off by the math, and few mathematicians were interested in the problems of biology. The following year, however, an ambitious entomologist named E. O. Wilson read the paper. Wilson wanted to understand the altruism at work in ant colonies, and he became convinced that Hamilton had solved the problem. By the late nineteen-seventies, Hamilton’s work was featured prominently in textbooks; his original papers have become some of the most cited in evolutionary biology. As Wilson realized, the equation allowed naturalists to make sense of animal behavior using genetic models, giving the field a new sense of rigor. In an obituary published after Hamilton’s death, in 2000, the Oxford biologist Richard Dawkins referred to Hamilton as “the most distinguished Darwinian since Darwin.” But now, in an abrupt intellectual shift, Wilson says that his embrace of Hamilton’s equation was a serious scientific mistake. Wilson’s apostasy, which he lays out in a forthcoming book, “The Social Conquest of the Earth,” has set off a scientific furor. The vast majority of his academic colleagues are convinced that he was right the first time, and that his recantation has damaged the field. The controversy is fuelled by a larger debate about the evolution of altruism. Can true altruism even exist? Is generosity a sustainable trait? Or are living things inherently selfish, our kindness nothing but a mask?