There have been two major population explosions in the course of human social evolution. By the end of the Paleolithic Period the world’s human population is estimated to have been between five and six million (an average of 0.1 person per square mile [0.04 person per square kilometre] of Earth’s land area). Following the Neolithic or agricultural revolution, the population made its first major leap, reaching over the short span of 8,000 years around 150 million by the year 1000 bce (2.6 persons per square mile). For the next two and a half thousand years there was relatively little change. World population had reached about 500 million by the middle of the 17th century. During this time any tendency for population to grow was punished by the checks of starvation and pestilence. Only with the Industrial Revolution of the 18th century did population growth break out again from its Malthusian fetters.

From about 1700 there was a second and far more rapid population explosion. Since the late 1600s the world’s population has increased more than 15-fold. This gives some measure of the difference between the two population revolutions of human history: there has been a dramatic increase not simply in population but in the rate of increase of population since industrialization took hold. Between 1650 and 1850 the average annual rate of increase of the world’s population doubled; it doubled again by the 1920s, and it more than doubled, once more, by the 1970s.

If the time taken to double the world’s population over the past 350 years is taken as a measure, then the doubling time is seen to have been shrinking fast. It took 200 years, to 1825, to double the world’s population from 500 million to 1 billion. It took only 100 years to achieve the next doubling, bringing the total to 2 billion by 1930, and only 45 years to achieve yet another doubling, to 4 billion by 1975. The world’s population 45 years later—about 7.7 billion by 2020—indicated a leveling off of the growth rate in the late 20th and early 21st centuries.

It was in western Europe, with the Industrial Revolution, that the second population revolution began. Europe’s population doubled during the 18th century, from roughly 100 million to almost 200 million, and doubled again during the 19th century, to about 400 million. It was in Europe, too, that the pattern first emerged that has come to be known as the “demographic transition” (see population: Theory of the demographic transition). The populations of nonindustrial countries are normally stable (and low) because high birth rates are matched by high death rates. With industrialization, improvements in medical knowledge and public health, together with a more regular food supply, bring about a drastic reduction in the death rate but no corresponding decline in the birth rate. The result is a population explosion, as experienced in 19th-century Europe. In time, however, as European societies showed in the early 20th century, the urbanized populations of industrial societies voluntarily lower their birth rates and population growth flattens out. A new population plateau is reached. Japan, industrializing some 50 years later than the West, provided an almost textbook demonstration of the pattern of the demographic transition. Its population grew rapidly after 1870, during its industrializing phase, and leveled off equally rapidly after World War II. In an even more speeded-up form, Russia and the Soviet Union, in their century of industrialization that began in the 1880s, illustrated the link between industrialization and population.

Does the demographic transition hold good for developing societies? Nearly all of these countries experienced rapid population growth after World War II, at rates greater than had ever occurred anywhere in the West. Western aid and medical science spectacularly reduced the high death rates, often by more than 50 percent. Determined population-control efforts in a few countries, such as Singapore, India, and China, yielded clear results. Only in Africa did population continue to rapidly grow into the 21st century. One important characteristic of societies that have not yet undergone a demographic transformation is the persistence of predominantly youthful populations, though these societies can least afford the burden of feeding and educating their nonproductive young.

It was argued that the birth rate remained stubbornly high in these societies partly because industrialization was so slow and fragmentary in the developing world. In addition, where any significant development had taken place, as in Brazil or Malaysia, it had only really affected a small elite; the mass of the people were untouched. Thus, the reasons people in the industrialized West chose to have fewer children lacked cogency in underdeveloped countries. It remained rational for the bulk of the population to continue to have large families both to share in manual labour and to provide security for parents in their old age. Lower fertility would come, it was argued, when wealth was more evenly distributed and social security systems well established.