How Do We Know the Facts of Demography?

Demographers know that a population that is increasing slowly has a higher proportion of old people than one that is increasing rapidly; and that differences in birth rates have a larger influence on the age distribution than do differences in death rates. They also often claim that a poor country whose population is growing rapidly will increase its income per head faster if it lowers its birth rate than if it maintains a high birth rate. How do demographers know these things? Many readers will be surprised to learn that in a science thought of as empirical, often criticized for its lack of theory, the most important relations cannot be established by direct observation, which tends to provide enigmatic and inconsistent reports. Confrontation of data with theory is essential for correct interpretation of such relationships, even though on a particular issue it more often generates an agenda for further investigation than yields useful knowledge. This article will examine how demographers distill knowledge from observation and from theory. It also will try to show how a reigning theory can be successfully challenged. Let no one think these questions are remote or purely abstract. The resolution of the major policy issues of our time depends on the answers. How much of their development effort should poor countries put into birth control if they deem their rate of population growth excessive? Some would put nothing, in the expectation that rapid increase of income will by itself bring population under control. Once people have automobiles, once their countryside is paved over with roads, once enough air-conditioned houses