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At the time, Trivers knew little about evolution and nothing at all about biology, but he plunged into the literature and sought out mentors. "Once I learned what natural selection was," he says, "it was clear that for one hundred years since Darwin, almost no work had been done in applying Darwin's reasoning to social behavior. It was an incredible opportunity to be able to move into this enormous vacuum."
Excited by his new interest, Trivers borrowed money and went back to Harvard as a special student in biology, gaining his Ph.D. and a faculty appointment in 1972. Zoologist Ernest Williams, one of his teachers, describes him as a brash, brilliant student who turned in papers with slashing attacks on well-known biologists, some of whom have not forgotten—or forgiven. Brashness is still part of Trivers' character. He derided an anthropologist (who, incidentally, admires his work) as too old to understand the implications of sociobiology. The anthropologist was then 38.
The second of seven children, Trivers admits that the problems of growing up in a large family and the arguments he had with his father helped to point him toward his theory that parent-child conflict is biologically certain. Trivers believes that the child shows a selfish interest in itself and seeks to get more than its fair share of the energy and resources of parents. But the parent has only a partial genetic interest in each child and thus is preoccupied with sharing resources. The result, according to Trivers, is biologically certain conflict between the child, who tends toward selfishness, and the parent, who insists that the child share.
Another example of the conflict, in a variety of mammals, is weaning. When the benefit to the child begins to be outweighed by the cost to the mother (reduced ability to bear or care for other offspring), the mother will deny milk, though the offspring will continue to demand more. But parents have an edge. (Says Trivers: "An offspring cannot fling its mother to the ground at will and nurse.") So evolution has provided a defensive weapon for the offspring: psychological warfare. Some fledgling birds will scream with hunger—even when they are reasonably well fed—to induce the parent to bring more food. Dogs withhold tail-wagging to get more food. Children withhold or provide smiles—as a means of reinforcing maternal behavior they need. Says Trivers: "Strong selection pressures tend to favor the infant's efforts to express its own self-interest. Once you explore the stratagems of parent and child, I think you can see that the child is not just an empty vessel to be filled by the parents but a sophisticated organism capable of acting in its own self-interests from early on."
So early, Trivers thinks, that the action may actually begin before birth. He believes there are "chemical tactics" that the fetus uses on the mother to increase its size and fitness while still in the womb. Even more surprising is Trivers' theory (for which he admits there is yet no evidence) of genetic conflict between egg and sperm before conception: under some conditions, the egg may try to repel sperm with female-producing X chromosomes in order to be fertilized into a boy rather than a girl.