Excerpt from
The Handbook Of Evolutionary Psychology (2005), edited by David Buss:
Comparative primate studies sometimes indicate that humans are designed for monogamy. Among the monogamous white-handed gibbon (Hylobates lar), the average body weight of an adult male is about 1,000 times the weight of the average male's testes (Dixson, 1998). Among humans, the average man's body weight is about 1,300 times the size of the average man's testes (Schultz, 1938), a ratio similar to the white-handed gibbon. In contrast, the more short-term-orientated common chimpanzee (Pan troglodytes) possesses extremely large testes with a body-testes ratio of only 350 (Dixson & Mundy, 1994), and the polygynous gorilla (Gorilla gorilla) has small testes with a body-testes ratio of over 5,000 (Hall-Craggs, 1962). Contradictory evidence regarding mating strategies exists in comparisons of primate seminal volume, sperm structure, and sperm quality (Baker & Bellis, 1995; Dixson, 1993; Moller, 1988). Overall, Dixson (1998) concluded that human male reproductive physiology is consistent with both monogamous and polygynous mating, providing only mixed support for the view that humans are monogamous.
Humans display extreme levels of altriciality compared to other primates, requiring large parental investments and possessing a relatively delayed adolescence (T.M. Mueller, 1999). Mate desertion is generally associated with lower infant survival in foraging cultures (Hill & Hurtado, 1996), another indication that humans are designed for monogamy. Finally, humans possess several neurophysiological systems of attachment linked with pairbonding and monogamy across species (Fisher, 1998; Hazan & Zeifman, 1999; Young, 2003).
Fisher (1992) suggests that human patterns of weaning, birth spacing, divorce, and remarriage all point to a system of serial monogamy. It takes about 4 years to wean a child in hunter-gatherer cultures, and birth spacing in a foraging environment averages about 4 years (Blurton Jones 1986). Many divorces occur between the fourth and sixth year of marriage (Fisher, 1989, 1992), and men who practice serial monogamy are more reproductively successful than men who stay married to the same woman for a lifetime. Women who mate serially do not have reproductive advantage over other women (Buckle, Gallup, & Rodd, 1996).
So, we are biologically wired to be monogamous for upwards of four years, which is when the bonding effect of oxytocin withers away, but there are competing evolutionarily stable strategies: alpha males (or whatever you want to call it) have the capacity for multiple polygynous partners, whereas beta males for monogamy. While women can entertain the interests of many suitors simultaneously, the story changes once she has sexed a guy after a few bangs. Women have much more to lose, in terms of provisioning, because they have the need to secure the providership resources of the best guy they can find. It’s very rare for a guy to possess both good genetics and good dad skills, so the Hobson’s Choice leads to a mating schedule of banging alpha cads in their youth, and then securing a beta male for provisioning.
There’s also the fun of sperm wars.