Natural Selection and Social Theory: Selected Papers of Robert L. Trivers

In early 1971 I decided to work on what I thought of as a biological theory of the family, a theory that would have variables such as sex and age and other relevant parameters and would derive how natural selection was acting on members of the family. There was only one problem: individuals in a family were related to each other, and somehow you had to take this relatedness into account when describing natural selection acting on the participants. I puzzled about the matter for some time and, in my usual style, consulted with advanced graduate students and relevant faculty, but without getting any help. What surprises me so much about this, in retrospect, was that I already knew Hamilton's kinship theory—he had, in fact, solved the very problem that was bedeviling me, and I had by then lectured on his work. But for some reason I still narrowly conceived his work as explaining altruistic traits per se, not as covering all interactions between kin. It was shortly after lecturing on the topic that it occurred to me that the key issue had already been solved, but I know that I greeted this discovery with relief. Hamilton had solved the difficult problem; the rest was easy. The key parameter turned out to be r, or degree of relatedness, the chance that one individual shares an identical copy of any given gene with another individual by direct descent (typically, 1/2 in both directions for parent–offspring). I narrowed the paper to mother–offspring conflict, as I had first called it, because I was thinking about mammals, but then later, in the usual syle, made the argument more general, changing it to parent–offspring conflict. (When my father saw the change in the title, he muttered to himself “I was afraid of that. ”)

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