DISCOUNTING immigration and emigration, changes in population are primarily a function of the difference between birth and death rates. For by far the greater period of human existence birth and death rates were high and the population of humans relatively stable. With the development of agriculture and technology, death rates fell and the population grew rapidly.

But following industrialisation birth rates (as reflected in average family size) also began to fall. Population growth in “developed” countries began to slow. In some counties today, as in the 1930s, birth rates are below “replacement” level and populations have started to decline.

The whole process is known as the “demographic transition.” It’s drivers are still a matter of research and debate but go well beyond the availability of contraception. They include factors such as the social and employment status of women, that children are no longer seen as an economic asset (additional “hands”
to work on the farm or in a factory) and the spread of what were initially “middle class” family norms in relation to consumption and lifestyle. The whole process is complicated still further by other factors, particularly religion.