John Ziker was a co-author of a study, “Greater wealth inequality, less polygyny: rethinking the polygyny threshold model,” published in July in Journal of the Royal Society: Interface, a publication from the Santa Fe Institute’s Dynamics of Wealth Inequality Project.
The study, to which Ziker contributed data from his work with indigenous hunter-gatherers in Siberia, revises the standard polygyny threshold model which only allows for female choice, to a model in which both women and men make marital choices to maximize the number of surviving children they raise.
One of the unique contributions of this model is that it includes parameters reflecting diminishing fitness returns to increasing number of wives; rival wealth, defined as wealth that can be divided among wives; and finally non-rival wealth, like genetic quality which cannot be divided. The model was tested with data from 11,813 individuals’ wealth, marriage, and reproductive success in 29 diverse societies across the globe.
The standard polygyny threshold model predicts that polygyny should be more common and more pronounced in populations in which males differ substantially in their ability to control critical rival resources. A classic example is among elephant seals, when males fight to leave one (the alpha male) in control of beaches (rookeries) needed by females to mate and give birth. The larger or better the rookery the standard model predicts more females will choose to mate with that male. Among humans, the standard polygyny threshold model predicts that polygyny will be more common in socio-cultural contexts where wealth is held predominately by men, and where there is high inequality in its distribution.
There is a paradox in human societies though because as wealth inequality increases beyond a certain point, especially in populations employing intensive and industrial agriculture, the incidence of polygynous marriage decreases rather than increases, as predicted. Rather, polygyny is more common in the lowest tech “middle-range” societies centered on shifting agricultural or pastoral modes of production than the highest tech complex societies where wealth inequalities are the greatest and growing rapidly.
The paradox is resolved with the new model that considers both the effect of wealth inequality and the diminishing marginal value of additional wives. There are a number of alternative hypotheses to the shift to monogamy including the cultural selection of monogamous mating norms, however, these alternatives do not appear to be sufficient to explain the polygyny paradox with this new model and comparative study.