In March 2026, three prominent thinkers died within a day of each other. Lavish obituaries immediately marked the deaths of the always-wrong environmentalist Paul Ehrlich and the often-obscure political philosopher Jürgen Habermas. But two weeks after the death of Robert Trivers, one of the greatest evolutionary biologists since Charles Darwin, not a single major news source has noticed his passing. This despite Trivers’s singular accomplishment of showing how the endlessly fascinating complexities of human relations are grounded in the wellsprings of complex life. And despite the fact that the man’s life was itself an object of fascination. Trivers was no ordinary academic. He was privileged in upbringing but louche in lifestyle, personally endearing but at times obstreperous and irresponsible, otherworldly brilliant but forehead-slappingly foolish. 

Trivers’s contributions belong in the special category of ideas that are obvious once they are explained, yet eluded great minds for ages; simple enough to be stated in a few words, yet with implications that have busied scientists for decades. In an astonishing creative burst from 1971 to 1975, Trivers wrote five seminal essays that invoked patterns of genetic overlap to explain each of the major human relationships: male with female, parent with child, sibling with sibling, partner with partner, and a person with himself or herself. 

The fallout for science was vast. The fields of sociobiology, evolutionary psychology, behavioural ecology, and Darwinian social science are largely projects that test Trivers’s hypotheses. The ideas took pride of place in E. O. Wilson’s Sociobiology in 1975, Richard Dawkins’s The Selfish Gene in 1976, and many other bestsellers in the next three decades such as Robert Wright’s The Moral Animal (1994) and my own How the Mind Works (1997) and The Blank Slate (2002). In 2007 the ideas earned Trivers the Crafoord Prize, the equivalent of a Nobel for fields not recognised by Nobels. 

The fallout for our understanding of ourselves is also vast. The insight that partial genetic overlap among individuals leads to both confluences and conflicts of interests explains why human life is so intricate—why we love, and bicker with those we love; why we depend on one another, and mistrust those we depend on; why our emotions are powered by moral themes and not just physical threats; why deluded people are certain of their convictions and evil ones convinced of their rectitude. As Trivers put it, 

Darwinian social theory gives us a glimpse of an underlying symmetry and logic in social relationships which, when more fully comprehended by ourselves, should revitalize our political understanding and provide the intellectual support for a science and medicine of psychology. In the process it should also give us a deeper understanding of the many roots of our suffering.

The ground for Trivers’s revolution had been laid in the 1960s by George Williams and William Hamilton, who reminded their fellow biologists that natural selection is driven by competition among replicators. This implies that the beneficiary of evolutionary adaptations—what an organ or an instinct is for—is not the group or even the individual, as folk understanding had it, but the gene. Hamilton drew out an implication: genes can perpetuate themselves by nurturing not only offspring but siblings and other kin, since any genes that benefit a blood relative would, with a certain probability, benefit copies of themselves in the bodies of those relatives. 

Trivers’s innovation was to show how the partial overlap of genetic interests between individuals should put them in a partial conflict of psychological interest. The key resource is parental investment: the time, energy, and risk devoted to the fitness of a child. Parents have to apportion their investment across all their children, each equally valuable (all else the same). But although parents share half their genes with each child, the child shares all its genes with itself, so its interest in its own welfare will exceed that of its parents. What the parent tacitly wants—half for Jack, half for Jill—is not what Jack and Jill each want: two thirds for the self, one third for the sib. Trivers called the predicament parent-offspring conflict. Its corollary is sibling-sibling conflict: every offspring has an interest in its siblings’ welfare, since it shares half its genes with the sib, but that interest is outweighed by the twofold genetic interest it has in itself. 

Jill may not even exist yet for Jack and his parents to differ over her welfare. Baby Jack may want to suck his mother dry, while Mom wants to keep part of herself in reserve for unborn Jill and other future offspring. The conflict is waged throughout the lifespan: in ailments of pregnancy (like preeclampsia and gestational diabetes), postpartum depression, infanticide, cuteness, weaning, brattiness, tantrums, rebelliousness, sibling rivalry, and struggles over parental attention, support, and inheritance. At least since Cain and Abel, family struggles have been a feedstock for fiction. Inventories of the plots that recur in stories worldwide invariably list “rivalry of kinsmen” or “enmity of kinsmen.”

Parental investment, Trivers explained, is also the ultimate casus belli in the battle of the sexes. When Darwin introduced the concept of sexual selection, he observed that in most species, males compete and females choose, but he had no idea why. Trivers explained the contrast by noting that in most species the minimal parental investments of males and females differ. Males can get away with a few seconds of copulation; females are on the hook for metabolically expensive egg-laying or pregnancy, and in mammals for years of nursing. The difference translates into differences in their ultimate evolutionary interests: males, but not females, can multiply their reproductive output with multiple partners. Darwin’s contrast can then be explained by simple market forces. And in species where the males invest more than the minimum (by feeding, protecting, or teaching their offspring), males are more vulnerable than females to infidelity (since they may be investing in another male’s child) and females are more vulnerable to desertion (since they may bear the costs of rearing their mutual offspring alone). 

Men typically invest more in their children than the males of other species, making our sexual choice and competition less lopsided. But the remaining conflicts of interest, rooted in the minimum that each sex can get away with, mean that sex is not just a natural source of mutual pleasure but rather takes place in the shadow of exploitation, illegitimacy, jealousy, spousal abuse, cuckoldry, desertion, harassment, and rape. Even in a monogamous lifelong couple, whose reproductive fates are bound together in their common children, an ever-present potential for conflict exists in the tug of stepchildren and in-laws, in the temptations of adultery, and in the possibility that one will die first. This mixture of overlap and divergence leads to the endless passions explored in gossip, story, and song, and played out in life. George Bernard Shaw wrote, “When we want to read of the deeds done for love, whither do we turn? To the murder column.”