Many evolutionary biologists believe that men used to be largely polygamous. Ernst Mayr (1970, p. 386) writes:
Among Alaskan Eskimos, among New Guinea mountain Papuans, and among relatively untouched South American Indians, polygamy is widespread, and it is the individual with leadership qualities who has the greatest chance to have several wives.
He then goes on to quote J.V. Neel’s statement that “it may be that the single most dysgenic event in the history of mankind was departure from a pattern of polygamy based on leadership, ability, and initiative.” Mayr concludes that polygyny (i.e., male polygamy) drove the increase in human brain size and that a shift to monogamy explains why this increase came to a halt some 100 thousand to 200 thousand years ago.
This viewpoint has several weaknesses. First, if modern humans began spreading out of Africa only 50,000 or so years ago, one can hardly point to Eskimos, New Guineans, and South American Indians as examples of human behavior before 100,000-200,000 BP.
Second, polygyny does intensify selection of men, but it also relaxes selection of women. Indeed, it greatly increases the reproductive opportunities of low-status women who, otherwise, might not reproduce. Polygynous men usually recruit their additional wives from the lower ranks of society; this is, after all, where they can elbow out low-status rivals. In fact, out of all mating systems, monogamy is arguably the one most conducive to natural selection, since it curbs ‘marrying up’ and condemns most low-status individuals to eventual genetic death (their places being taken by downwardly mobile descendants of higher-status individuals). This is the argument that Gregory Clark has recently made in A Farewell to Alms.
Third, before 10,000 years ago all humans lived by hunting and gathering and among present-day hunter-gatherers the polygyny rate is low: 9.5% of all sexual unions among the Inuit in general (Kjellstrom, 1973, pp. 114-115); 6% among !Kung Bushmen (Howell, 1979, p. 235); 5% among Eastern (Ituri) Pygmies (Turnbull, 1986, p. 111); 16.6% among Western Pygmies (Cavalli-Sforzi, 1986, p. 37); and 15% among central Australian Aborigines (Birdsell, 1993). Only with the advent of agriculture, especially tropical agriculture, do these rates enter the 20%-50% range, notably in sub-Saharan Africa and Papua-New-Guinea. Year-round farming enables women to become self-reliant in feeding themselves and their children, thus making it less costly for men to take second wives.
Some have argued that present-day hunter-gatherers now live in resource-poor environments and are thus unrepresentative of ancestral hunter-gatherers. Among other things, this means that some ancestral environments may have favored high food self-sufficiency among women and correspondingly high polygyny rates among men. If true, this argument simply drives home the point that modern humans did not live in a uniform environment of evolutionary adaptedness. Some parts of the world had more polygyny than did others.
Among hunter-gatherers, generally speaking, polygyny was costlier for men with increasing distance from the equator. Longer winters restricted food gathering and made women depend more on their mates for provisioning (Frost, 2006; Kelly, 1995, pp. 262-270; Hoffecker, 2002, p. 8; Martin, 1974, pp. 16-18). In the Arctic, where women had almost no opportunities for gathering, only the ablest hunter could provide for a second wife (Kjellström, 1973, p. 118).
Birdsell, J.B. (1993). Microevolutionary patterns in Aboriginal Australia: A gradient analysis of clines. New York: Oxford University Press.
Cavalli-Sforza, L.L. (1986). Demographic data. In (L.L. Cavalli-Sforza ed.). African Pygmies, pp. 23-44. Academic Press.
Clark, G. (2007). A Farewell to Alms: A Brief Economic History of the World. Princeton (NJ): Princeton University Press.
Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103
Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.
Howell, N. (1979). Demography of the Dobe !Kung. New York: Academic Press.
Kelly, R.L. (1995). The Foraging Spectrum. Diversity in Hunter-Gatherer Lifeways. Washington: Smithsonian Institution Press.
Kjellström, R. (1973). Eskimo Marriage. An Account of Traditional Eskimo Courtship and Marriage. Lund: Nordiska Museets Handlingar 80.
Martin, M.K. (1974). The Foraging Adaptation — Uniformity or Diversity? Addison‑Wesley Module in Anthropology 56.
Mayr, E. (1970). Populations, Species, and Evolution. Cambridge (Mass.): Harvard Press.
Turnbull, C.M. (1986). Survival factors among Mbuti and other hunters of the equatorial African rain forest. In (L.L. Cavalli-Sforza ed.). African Pygmies, pp. 103-123. Academic Press.