Population increase is apparent even in the Paleolithic (foraging) era, though in this era it mainly took the form of a slow but accelerating capacity to migrate into new environments. Humans probably evolved in the savanna lands of southern and eastern Africa, and for all but the last ten thousand years, we have lived as foragers. But the variety of foraging techniques used by humans clearly increased over time. Archaeological evidence suggests that even 100,000 years ago, humans were exploring new environments—in arid regions, for example, or near seashores or in tropical forests (McBrearty and Brooks 2000, 493–494, 530). Then, from about 100,000 years ago, humans began to migrate out of Africa. They appeared in the very different environments of Australia (from perhaps fifty thousand years ago), ice age Siberia (perhaps thirty thousand years ago), and finally the Americas (from at least thirteen thousand years ago). Though we have no direct evidence, we can be sure that these migrations meant an increase in the total number of humans. This remains true even though each particular community remained small, so that contemporaries could hardly be aware that human numbers were increasing.

An aerial view of Levittown, Pennsylvania, circa 1959. Numerous “Levittowns,” the first American suburbs, sprang up after World War II to accommodate the population surge known as the baby boom.

From about ten thousand years ago, as the last ice age was ending, agricultural communities appeared in different parts of the world. Agriculture quickened population growth. Current estimates suggest that there may have been 5 to 10 million humans ten thousand years ago; 50 million five thousand years ago; and 250 million just one thousand years ago. Agriculture stimulated population growth in many ways. Nomadic foragers have a limited ability to increase production from a given area, so they have powerful reasons to limit the number of children too young to walk or forage on their own, and to limit the numbers trying to feed from a given area. Modern anthropological studies suggest that foragers have many ways of limiting population growth, including prolonged breast feeding and even infanticide. But agriculturalists are normally sedentary, so transporting young children is not such a problem. Furthermore agriculture makes it possible to increase the amount of food produced within a given area, so it is possible to accommodate population increase even without migrating to new lands. Perhaps even more important, for farming households in most pre-modern agricultural communities it was important to have lots of children because this increased the available labor. In a world of high death rates (commonly in pre-modern agricultural societies, up to 20 percent of infants died in their first year, and another 30 percent before their fifth birthday), the best way of maximizing the number of children surviving to adulthood was to have as many children as possible. Such behavior, and the increased resources available within agrarian societies, ensured that populations would grow much faster than in the Paleolithic era.

As in the Paleolithic era, population growth led to migrations, and as farmers migrated they helped spread agriculture around the world. But migration was not the only option, for farmers, unlike foragers, could also create larger and more densely settled communities. Eventually, “intensification” of this kind led to the emergence of communities large enough and complicated enough to be called cities. Seeing these long-term trends was not always easy at the time, because local population gains could easily be wiped out by famines or epidemics. So, to contemporaries, cycles of growth and decline were more apparent than the long-term trend toward growth. Indeed, these cycles provide the central idea of Thomas Malthus (1766–1834), the founder of modern demographic studies: that population growth would always end up exceeding productive capacity so that periodic population crashes were inevitable.

In the last thousand years, population growth has accelerated once again as the global human population has risen from 250 million one thousand years ago, to 950 million two hundred years ago, and about 6.8 billion in 2010. There are many specific causes for population increase in given instances, but once again, the most general explanation for this increase is that increasing rates of innovation have allowed humans to produce the food and resources needed to support rapid population growth. Death rates fell in many parts of the world in the last two centuries, partly because of the spread of new crops and improved knowledge about basic sanitation. In the twentieth century, scientific medicine and the introduction of antibiotics have reduced death rates throughout the world. But despite such advances, populations could not have continued to rise if it had not been for the burst of innovation we associate with the industrial revolution, which provided the energy, the resources, and the techniques needed to feed, clothe, and supply the rapidly increasing populations of the modern era. Some of the most important new technologies included improved forms of irrigation, the introduction of artificial fertilizers and pesticides, the use of fossil fuels in agriculture, and the breeding of new and more productive varieties of major food crops such as rice and maize. Equally important was the huge increase in available energy supplies made possible by the fossil fuels revolution.

The entrance to a Chinese apartment on a crowded block, June 2004. Multi-storey apartment complexes replace traditional hutong courtyards as China’s urban population continues to grow.

Today we may be in the middle of a new transition, often referred to as the “demographic transition.” Evidence has been accumulating, in some regions for more than a century or two, that as death rates fall and humans become more affluent, more urbanized, and better educated, they have fewer children. By 2000, there was no net population growth in more than thirty of the world’s more affluent countries. This trend seems to mark a return to a regime of systematic population limitation, like that of the Paleolithic era.

Although population growth has been a consequence of innovation, it has also shaped human history in profound ways. The size and density of populations can have a profound effect on rates of innovation, as well as on the structure of society, the power of states, the spread of disease, the health of commerce, and the availability of labor. The following are some of the major types of change in which population growth plays a significant role.

We have seen already that population growth can stimulate migration by causing localized overpopulation. Such migration, in its turn, has been one of the major causes of the spread of agricultural communities throughout the world. The rough calculations of the political scientist Rein Taagepera suggest that the area of the inhabited Earth within agrarian civilizations (a very rough measure of the area dominated by farming) grew from 0.2 percent some five thousand years ago to about 13 percent two thousand years ago, and perhaps 33 percent just three hundred years ago (Christian 2004, 305). While population growth was often the primary trigger for migrations of farmers into new regions, states often backed such migrations because their own fortunes depended on the size of the tax-paying populations under their control. State-backed migrations of this kind have been particularly spectacular in recent centuries as imperial states such as Russia, China, Spain, and Great Britain backed migrations of farmers into previously unfarmed lands in Siberia, Asia, the Americas, and Australasia.

Population growth has been one of the main drivers of increasing social complexity. There is little evidence for significant increases in the size or complexity of human communities until very late in the Paleolithic era. But human societies began to change significantly as soon as population densities started to rise with the appearance of agriculture. Larger, denser communities could no longer be organized through the informal kinship structures that worked well enough in foraging communities, most of which consisted of just a handful of people. Once hundreds or even thousands of people began to live together in villages and towns, new types of social coordination became necessary to limit conflict, organize worship, and organize collective activities such as defense, or the maintenance of irrigation systems. Eventually larger communities, supported by more productive technologies, allowed for the beginnings of specialization. No longer was everyone a farmer; so specialists had to buy their food and other necessities through markets, which had to be regulated and protected. Cities required even more sophisticated forms of coordination to manage refuse collection and maintain water quality, and for defense. The earliest states appeared, in part, to take over the new organizational tasks that emerged within dense communities; but their power also reflected the large numbers of people they controlled and taxed. In the last two hundred years, population growth has created new and even more complex human communities, so that modern states have had to acquire new administrative and technological skills to manage, tax, and coordinate the activities of many millions of people. All in all, increasing population density counts as one of the main drivers of social complexity in human history.

Population growth and the human migrations that resulted from it have also been a major cause of ecological degradation in human history. There is increasing evidence that this was true even in the Paleolithic era as communities of foragers developed technologies that had a significant impact on their environments. It seems likely that the arrival of humans led to the extinction of many species of large animals, particularly in Australia, Siberia, and the Americas. In Australia and the Americas, it is possible that 70 to 80 percent of all mammal species over 44 kilograms in weight were driven to extinction after the arrival of humans (Christian 2004, 200). But the slow spread of agrarian communities during the last ten thousand years has had a much more profound impact on the environment. Above all, it led to deforestation as farmers moved into forested zones, clearing trees as they went, at first temporarily and then more permanently. Overpopulation and overfarming occasionally caused regional breakdowns in production and the abrupt decline of entire regions. At the end of the third millennium bce, the populations of Mesopotamia, which lived mainly from sophisticated forms of irrigation farming, collapsed. The cause was probably over-irrigation leading to salinity, which undermined the productivity of the soil. The sudden decline of Mayan civilization at the end of the eighth century ce may also have been caused by overexploitation of fragile environments, leading to an abrupt decline in the fertility of the land.

Chinese one-child policy propaganda poster from 1986, by Zhou Yuwei. The caption reads, “Carry out family planning, implement basic national policy.”

Finally, though population growth has been a consequence of the human capacity for constant innovation, it can in its turn stimulate innovation by providing new markets and new incentives to increase productivity, and by increasing the number of people contributing to the pool of new ideas. Ester Boserup (1981), a scholar well known for her theories on economic and agricultural development, was a strong supporter of the view that population growth can stimulate innovation, particularly in agriculture. Yet it is clear that such arguments should not be overstated, for there have been all too many instances (some of which are listed above) when overpopulation did not generate new and more productive technologies, but led to social, economic, and demographic collapse. There seems to be a feedback loop between innovation and population growth. But it is not entirely clear which side of this feedback loop is most powerful, and the answer may vary from place to place. Was population growth a cause or a consequence of change? Theorists and governments have been divided on the issue, sometimes arguing for measures to support population growth, sometimes seeing overpopulation as a prelude to decline.

The debate has raged most furiously in discussions about the Industrial Revolution. Was population growth a cause or a consequence of innovation? There is no doubt that populations were growing rapidly in the eighteenth century in much of the world, or that population growth stimulated innovation by providing cheap labor and expanding markets for food and other basic necessities. Furthermore, in Britain growing populations increased the demand for fuel, which highlighted the shortage of wood and stimulated increasing use of coal, thereby encouraging the fundamental innovations (above all the steam engine) that led to increased use of fossil fuels. Yet it is also possible to argue that innovations in agriculture industry and commerce in turn led to further population growth. Global population growth was caused in part by the exchange of crops, animal species, and human migrants between Eurasia and the Americas (though the exchange of diseases between the continents caused a sharp decline in populations in the Americas). It has also been suggested that in many areas, both in Europe and elsewhere, the spread of new, commercialized forms of production and employment in rural areas (proto-industrialization) may have enabled young couples to create families at an even younger age, whereas previously they might have had to wait until enough land became available to set up a new household and start producing children. Population growth was also caused in part by an increase in the areas being farmed or exploited in new ways, both within major states and in newly colonized regions, usually with the backing of governments. All these factors—the emergence of global markets, the search for new resources with state backing, and the emergence of more industrialized forms of production in rural areas—may have stimulated population growth in the early modern period. In the nineteenth century, the industrial revolution itself stimulated population growth in new ways. New forms of sanitation reduced mortality rates, and artificial fertilizers increased food supplies. Eventually, modern scientific medicines and the spread of knowledge about sanitation helped drastically reduce mortality rates around the world.

Such discussions show that, however necessary it may be to separate out factors such as population growth as causes of historical change, it is always somewhat artificial to do so. The very notion of “engines of growth” is little more than a crude way of trying to clarify the relative importance of different causes of change. Throughout human history, long-term population growth has stimulated change; but the long demographic trends have themselves been shaped by technological innovations, climatic patterns, the actions of states, and the spread of disease, in a complex feedback cycle that needs to be analyzed instance by instance.

Agricultural Societies; Biological Exchanges; Birth Control; Carrying Capacity; Columbian Exchange; Engines of History; Globalization—Historical Perspectives; Globalization—Modern Conceptions; Migrations—Overview; Malthus, Thomas; Population and the Environment