Monogamy:

Richard D. Alexander, The Biology of Moral Systems;

"To the extent that males and females (of any species) commit themselves to lifetime monogamy, the interests of two individuals in a pair approach being identical. This point is often confused by biologists and social scientists alike (e.g., Dawkins, 1976, and Sahlins, 1976, both thought that unrelated spouses necessarily disagree more than relatives)."