Because sex ratios are a key factor regulating mating success and
subsequent fitness both across and within species, there is widespread
interest in how population-wide sex ratio imbalances affect marriage
markets and the formation of families in human societies. Although most
modern cities have more women than men and suffer from low fertility
rates, the effects of female-biased sex ratios have garnered less
attention than male-biased ratios. Here, we analyze how sex ratios are
linked to marriages, reproductive histories, dispersal, and urbanization
by taking advantage of a natural experiment in which an entire
population was forcibly displaced during World War II to other local
Finnish populations of varying sizes and sex ratios. Using a discrete
time-event generalized linear mixed-effects model, and including factors
that change across time, such as annual sex ratio, we show how sex
ratios, reproduction, and migration are connected in a female-dominated
environment. Young childless women migrated toward urban centers where
work was available to women, and away from male-biased rural areas. In
such areas where there were more females, women were less likely to
start reproduction. Despite this constraint, women showed little
flexibility in mate choice, with no evidence for an increase in partner
age difference in female-biased areas. We propose that together these
behaviors and conditions combine to generate an “urban fertility trap”
which may have important consequences for our understanding of the
fertility dynamics of today including the current fertility decline
across the developed world.