Throughout the twentieth century, the population of South Africa grew. Between Union and 1996, the total number of South Africans increased sixfold, from just under six million to just under thirty-eight million. Until 1948, the proportion of whites in the population remained more or less constant, at around 23 per cent, but then began to decline sharply, until by 1988 it was only around 14 per cent. (In 1996 the census did not concern itself with such classifications.) The proportion of coloureds and Indians remained fairly steady, except during the 1950s, which saw a rise of over one per cent in the coloureds' share, presumably as a result of stricter classification following the Population Registration Act, not of major demographic shifts.

The conclusion that can be drawn from these figures is that the black population continued to grow steadily and sharply. Between 1910 and 1948 it grew at, on average, 1.92 per cent per annum, and in the first 31 years of National Party rule at 3.14 per cent per annum. From the 1980s, and perhaps earlier, this rate of growth declined, until by the 1990s it was below 2 per cent.

At the macro level, the reasons for this population growth are quite clear. There have been no major famines or other crises of subsistence in the country since the rinderpest epizootic of the 1890s. The only major population crash was caused by the influenza pandemic of 1919. A steadily high birth rate has given way to a birth rate that has begun to fall, as, increasingly, the costs of bringing up children have come to exceed the benefits which could be gained from their labour. This was not realised sufficiently by the demographers, so that after 1970 the country's population was consistently overestimated, eventually by 10 per cent, or four million people.