A Simple Explanation of Tempo Effects in Fertility – and Why Such Issues are Irrelevant for Mortality Analysis

John R. Wilmoth, University of California, Berkeley

We propose a general definition of the tempo of demographic processes based on percentiles of age distributions of events (e.g., birth, death) and develop formal models of tempo change for both fertility and mortality within a common framework. Using these models we attempt to explain as clearly as possible the nature of tempo effects in fertility and to demonstrate the absence of comparable effects for mortality. One key observation is that in situations of tempo delay, cohorts spend increasing amounts of time passing between equivalent percentiles of a distribution, leading to longer waiting or survival times. Mathematically, these longer intervals have mirror images in the form of reduced demographic rates (assuming no change in cohort quantum). We show that the associated reduction in period fertility results entirely from stretching cohort reproductive intervals and misrepresents cohort quantum. However, a similar reduction in period mortality accurately reflects gains in cohort survival.