A country has just reached replacement-level fertility (TFR = 2.1), but its population continues to grow rapidly for several decades. What is the demographic mechanism behind this continued growth?
AReplacement-level fertility must be higher than 2.1 in this country due to high mortality
BImmigration is driving the growth, not natural increase
CThe large cohorts born during the high-fertility era are now entering their childbearing years, producing more births than the smaller elderly cohorts produce deaths — even at replacement-level fertility
DThe TFR measurement is incorrect; actual fertility must be above replacement
This is population momentum in action. The age structure is young — disproportionately concentrated in the childbearing ages — because of decades of high fertility. At replacement-level fertility, each woman has about 2.1 children, but there are many more women of childbearing age than elderly people dying. Total births exceed total deaths, and the population grows. This growth will continue until the age structure reaches the stationary distribution implied by replacement-level vital rates — a process that takes 50-70 years.
Question 2 True / False
Population momentum only operates in the positive direction — populations can have built-in future growth but not built-in future decline.
TTrue
FFalse
Answer: False
Negative momentum is real and increasingly relevant. A population that has had below-replacement fertility for decades develops an old age structure with relatively few women of childbearing age. Even if fertility were to rise instantly to replacement level, deaths would exceed births for decades because the small reproductive-age cohorts cannot produce enough births to offset the large elderly cohorts dying. Many European countries and Japan have negative momentum built into their age structures.
Question 3 Short Answer
Keyfitz's momentum formula gives the ratio of the eventual stationary population to the current population when fertility instantly drops to replacement. For a typical sub-Saharan African country, this ratio might be 1.5-1.8. Explain in demographic terms what this means and why it matters for policy.
Think about your answer, then reveal below.
Model answer: A momentum ratio of 1.5 means the population would grow by 50% above its current size before stabilizing, even under the most optimistic fertility assumption (instant replacement). A ratio of 1.8 means 80% additional growth is locked in by the age structure. This matters for policy because it means even successful fertility reduction programs cannot prevent substantial additional growth — the large young cohorts will have children regardless of the fertility rate per woman. Infrastructure, education, healthcare, and employment planning must account for decades of continued growth that no policy can prevent, only the magnitude of which can be influenced.
Momentum is perhaps the single most policy-relevant concept in demography because it reveals the limits of what fertility policy can achieve in the short and medium term. Countries that delay fertility decline lock in more momentum; those that achieve rapid decline lock in less. But some momentum is unavoidable once a population has experienced sustained high fertility, and planners who ignore it will systematically underinvest in services for the coming generations.