Aiming at a moving target: Period fertility and changing reproductive goals.

Summary Common sense suggests that changes over time in aggregate period fertility rates should be closely, related to changes in desired completed fertility after controlling for contraceptive failure, and desired spacing and timing; the nature of the relationship is, however, far from clear. This paper shows that when desired completed fertility undergoes swings, like those in the United States in recent decades, the turning points in period fertility will precede those in desired completed fertility by as much as five years and the amplitude of the swings in period fertility will be more than twice as great. Cumulated fertility, on the other hand, will lag behind reproductive goals. Period fertility rates will exceed desired completed fertility when desires are increasing and fall below it when desires are decreasing. These theoretical results help to explain some salient features of the American baby boom and bust. It is also shown that during a demographic transition, period fertility will fall more rapidly than desired completed fertility, and that towards the end of the transition, period fertility will increase.