Monogamy and Reproductive Opportunity Leveling:

"Alexander et al. (1979; see also Alexander, 1975) have argued that socially or legally imposed monogamy is a way of leveling the reproductive opportunities of men, thereby reducing their competitiveness and increasing their likelihood of cooperativeness. The imposition of monogamy by custom or law has the interesting effect of reducing both male-male and male-female conflicts to a minimum, especially when clans are discouraged (as in nation states: see Alexander, 1979, pp 256-259), and when married couples do not have differential access to their respective relatives (e.g., when they are "neolocal" or reside in a new locality rather than becoming a part of one or the other extended family of relatives). Moreover, the combination of socially or legally imposed monogamy, neolocality, and close association of the married couple in work not only leads to minimizing of philandering and conflict of interest between husband and wife, but also characterizes the largest (and perhaps the most unified - or durable - of all large) human societies. ..."

"the most extremely ultrasocial systems of humans and other species are apparently all based on reproductive opportunity leveling."

The Biology of Moral Systems
Richard D. Alexander (1987)