Culled Males, Infant Mortality and Reproductive Success in Historical Finland

Tim-Allen Bruckner, University of California, Irvine

Theory asserts that the sex ratio (i.e., M/F) at birth gauges selection in utero and cohort quality of surviving males. We report the first individual-level test in humans of the “culled cohort” hypothesis that males born to low sex ratio cohorts show lower than expected infant mortality but greater than expected reproductive success. We examined a unique multigenerational dataset in 19th century Finland (n=7,824 males). A one standard deviation decline in the cohort sex ratio precedes an eight percent decrease in male infant mortality. Males born to lower cohort sex ratios also successfully raised four percent more offspring to reproductive age than did males born to higher cohort sex ratios, but the offspring finding falls just outside conventional statistical significance. Whereas the sex ratio gauges selection in utero and predicts male infant mortality, the reproductive success findings provide weak support for an evolutionarily adaptive explanation of male culling in utero.