Abstract
Monogamy appears to have become the predominant human mating system with the emergence of highly unequal agricultural populations that replaced relatively egalitarian horticultural populations, challenging the conventional idea—based on the polygyny threshold model—that polygyny should be positively associated with wealth inequality. To address this polygyny paradox, we generalize the standard polygyny threshold model to a mutual mate choice model predicting the fraction of women married polygynously. We then demonstrate two conditions that are jointly sufficient to make monogamy the predominant marriage form, even in highly unequal societies. We assess if these conditions are satisfied using individual-level data from 29 human populations. Our analysis shows that with the shift to stratified agricultural economies: (i) the population frequency of relatively poor individuals increased, increasing wealth inequality, but decreasing the frequency of individuals with sufficient wealth to secure polygynous marriage, and (ii) diminishing marginal fitness returns to additional wives prevent extremely wealthy men from obtaining as many wives as their relative wealth would otherwise predict. These conditions jointly lead to a high population-level frequency of monogamy.
Highlights
Decades of both theoretical [1,2,3,4] and empirical research [5,6,7,8,9] based on the polygyny threshold model [1,2] have suggested that polygyny should be more common and more pronounced in populations in which males differ substantially in resource control
Most anthropological analyses of polygyny limit the definition of the term to two or more co-occuring wives of one man, we forego the sequential/concurrent distinction because (i) a male’s wealth is generally shared to some degree across all wives and the children of each over the male’s lifetime and (ii) as we show later, the elasticities2 of fitness with respect to times married are reliably positive for almost all populations sampled here, even those in which serial monogamy is practised
We demonstrate two theoretical results with the potential to resolve the polygyny paradox
Summary
Decades of both theoretical [1,2,3,4] and empirical research [5,6,7,8,9] based on the polygyny threshold model [1,2] have suggested that polygyny should be more common and more pronounced in populations in which males differ substantially in resource control In humans, this will be in socio-cultural contexts where wealth is held predominately by men, and where there is high inequality in its distribution. Since human behavioural variation is often determined by many underlying factors, there are likely to be complementary effects among the potential causes identified in these hypotheses
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