Delayed reproduction has unexpected effects on population growth and structure

It is widely accepted that delayed reproduction reduces the population growth rate, with associated effects on population structure and size. Policies (e.g., “later, longer, fewer”) have been based on this conclusion. However, it is rarely noted that the negative effect of reproductive delay on population growth applies to populations with positive growth rates. Many countries now experience below-replacement fertility levels and growth rates that would lead to population decline. In such populations, delayed reproduction increases, rather than decreases, population growth. This paper calculates the effects of delayed reproduction on the population growth rate, the population age distribution, and the equilibria of stationary-through-immigration populations. It does so for reproduction measured by age-specific fertility and by the parity transition matrix. In populations with below-replacement fertility, delayed reproduction leads to higher, not lower, population growth; to younger, not older, populations; and to larger, not smaller, equilibria. Examples are presented using age-specific rates for Japan and age × parity-specific rates for Slovakia; in both cases over a demographic transition from positive to negative growth.