All group-living nonhuman primates, such as chimpanzees and the lesser known bonobos, are polygamous. Perhaps not coincidentally, researchers have documented infidelity in every human culture. Yet, most evolutionary biologists agree that monogamy is natural to humans and that it has evolved to assure the survival of our species through guaranteed paternal child support. In other words, without monogamy there is no guarantee a guy would stick around to invest in his offspring. Christopher Ryan and Cacilda Jethá, authors of Sex at Dawn,1 argue that a driving force behind this assured “male parental investment” is the certainty that it’s the particular male’s genes that are passed on to any offspring in which he invests. A monogamous bond insures a man will not accidentally support another man’s child, while it simultaneously assures the female that her male partner will not share resources with another woman’s offspring.
If monogamy is so natural, however, then why is it that cultures need to sanction monogamy?