Fertility timing and the birth squeeze

Objective: To provide an in-depth examination of the birth squeeze, the alteration of male and female birth rates by the age-sex composition of the population. Methods: Mathematical analyses using reasonable simplifying assumptions are made to find male and female total fertility rates (TFRs), and from them an index of the magnitude of the birth squeeze under exponential, linear, and sinusoidal birth trajectories. Results: Explicit equations are shown for male and female TFRs and for birth squeeze measures under the three birth trajectories. In all cases, the magnitude of the birth squeeze was approximately proportional to (1) a meaningful measure of population change, (2) the difference between the male and female mean ages of fertility, and (3) the sex ratio at birth. Despite the strong influence of cycle length T in sinusoidal models, the cycle time of the maximum birth squeeze is consistently found near time 30, and the minimum near time 30 + T/2. Contribution: Birth squeezes are a potentially important, though rarely considered, factor in fertility analyses. Here, easy-to-apply methods are presented that show how male and female fertility rates are affected by population composition, and that measure the magnitude of birth squeezes. New, interpretable regularities are derived that relate birth squeezes to meaningful demographic measures and provide new insights into fertility dynamics.