We propose an explanation for almost a century of changes in U.S. marriage rates, in three
stages. First, we show that changes in cohort size alone can account for around 50 to 70% of
the variation in marriage rates since the 1930s for both black and white populations. Specifically,
increases in cohort size reduce marriage rates, whereas declines in cohort size have the opposite
effect. We provide the most convincing evidence on this relationship by using variation in cohort size
due to differences across states in sale bans on oral contraceptives. Using this exogenous variation
in access to oral contraceptives, and consequently the number of births, we provide evidence that
the relationship between changes in cohort size and changes in marriage rates is causal. Next, we
develop a dynamic search model of the marriage market that qualitatively generates this observed
relationship, and derive a testable implication about cohort size’s effect on spouses’ age differences.
Finally, we estimate the model and investigate its consistency with the data. We fail to reject it
using the derived implication, and find that it can quantitatively explain much of the observed
variation in marriage rates.