Mammals are not big on monogamy. In fewer than 10 percent of species is it common for two individuals to mate exclusively. The primate wing of the group is only slightly more prone to pairing off. Although 15 to 29 percent of primate species favor living together as couples, far fewer commit to monogamy as humans know it—an exclusive sexual partnership between two individuals.
Humans obviously have an imperfect track record. People have affairs, get divorced and, in some cultures, marry multiple mates. In fact, polygamy appears in most of the world's societies. Yet even where polygamy is permitted, it is the minority arrangement. Most human societies are organized around the assumption that a large fraction of the population will pair off into enduring, sexually exclusive couples. And monogamy seems to have done our species good. “Pair bonds,” as scientists call monogamous relationships, were a crucial adaptation that arose in an archaic forebear that became central to human social systems and our evolutionary success. “We have a very big advantage over many other species by having pair bonds,” says University of Montreal anthropologist Bernard Chapais.