Pair bonding (or monogamy) is an extremely rare mating system among mammals, found in less than 5% of species (Kleiman, 1977). Nonetheless, it appears to be a central element in humans’ reproductive repertoire. It is therefore a curious fact that our dominant mating system is more like the typical mating system of birds than that of most mammals, including our nearest relatives, the Great Apes. In making this claim, it is important to be clear about three things. First, the claim is not that pair bonds necessarily last for life. In the absence of socially enforced lifelong monogamy, most pair bonds last for months or years but ultimately dissolve (Fisher, 1992). Note, though, that a significant minority of pair bonds do last until the end of the lifespan, even in traditional forager societies that lack rigid strictures on divorce (see,  e.g., Marlowe, 2004).

Second, the claim is not that human pair bonds are always sexually exclusive. Most surveys suggest that considerably fewer than 50% of men or women in long-term committed relationships are ever unfaithful (Blow & Hartnett, 2005). Nonetheless, some are, and as a result, a certain fraction of offspring are sired by someone other than the social father (the best estimates place this at around 1–3%; Anderson, 2006; Wolf, Musch, Enczmann, & Fischer, 2012). Third, the claim is not that pair bonding is our one “true” or natural mating system. Humans exhibit all the mating systems found in other species, including monogamy, polygyny (one man, two or more women), and even polyandry (one women, two or more men; Murdock, 1967).

It is also not uncommon for people to engage in extrapair mating, or to engage in casual sex before marriage or between long-term relationships. Different frequencies of each of these mating behaviors are found in different cultures and different historical periods. However, with the exception of long-term polyandry, all are relatively common, and thus all are plausibly part of the evolved repertoire of the human animal. Thus, our claim is not that pair bonding is humanity’s singular mating pattern. Our claim instead is simply that the pair bond is the most common setting for sex and reproduction in our species, that it has been for a long time, and that this has left a deep imprint on our evolved nature.

Psychological Inquiry

24:3, 137-168, (2013) DOI: 10.1080/1047840X.2013.804899
Steve Stewart-Williams & Andrew G. Thomas

This article looks at the evolution of sex differences in sexuality in human beings and asks whether evolutionary psychology sometimes exaggerates these differences. According to a common understanding of sexual selection theory, females in most species invest more than males in their offspring, and as a result, males compete for as many mates as possible, whereas females choose from among the competing males. The males-compete/females-choose (MCFC) model applies to many species but is misleading when applied to human beings. This is because males in our species commonly contribute to the rearing of the young, which reduces the sex difference in parental investment. Consequently, sex differences in our species are relatively modest. Rather than males competing and females choosing, humans have a system of mutual courtship: Both sexes are choosy about long-term mates, and both sexes compete for desirable mates. We call this the mutual mate choice (MMC) model. Although much of the evolutionary psychology literature is consistent with the MMC model, the traditional MCFC model exerts a strong influence on the field, distorting the emerging picture of the evolved sexual psychology of Homo sapiens. Specifically, it has led to the exaggeration of the magnitude of human sex differences, an overemphasis on men’s short-term mating inclinations, and a relative neglect of male mate choice and female mate competition. We advocate a stronger focus on the MMC model.