Local Scarcity of Women Predicts Higher Fertility among Married Couples and More Single Father Households

  • Daniel J Kruger University of Michigan
  • Sarah B Vanas University of Michigan
Keywords: sex ratio, fertility, sex differences, single parenthood


Because the reproductive strategies of men and women are somewhat divergent, influences of the sex ratio on the intensity of mating competition and selectivity for partners produce different outcomes in female biased and male biased populations. Male mating opportunities are enhanced by scarcity and incentives for long-term commitment are diminished, encouraging serial and simultaneous polygyny. Paternal investment is lower in these populations, as indicated by higher divorce rates, more out-of-wedlock births, and a greater proportion of single mother households. Scarce females are able to more effectively secure commitment from partners as well as demand higher levels of resource investment. Women marry earlier in these populations. Although single father households are relatively uncommon, we expect to see higher proportions of households with children headed by single fathers where women are scarce. We also expect to see higher fertility among married couples, both because women may have greater bargaining power in reproductive decision-making and the role of woman in childbearing may be more salient to and valued by men. Data from the 2000 U.S. Census across 318 Metropolitan Statistical Areas supported these hypotheses.

Author Biographies

Daniel J Kruger, University of Michigan
Daniel J. Kruger is a Research Assistant Professor at the School of Public Health and Institute for Social Research at the University of Michigan. His evolutionary research interests include: altruism, cooperation, competition, demography, life history, literary Darwinism, mortality patterns, risk taking, and interventions for social and ecological sustainability.
Sarah B Vanas, University of Michigan
Sarah Vanas is a participant in the Undergraduate Research Opportunities Program
Original Articles