By 1950, in most of the underdeveloped world, mortality had fallen to about half its pre-modern rate. The birth rate, however, had remained high and, by 1950, was about twice the death rate. For the rest of the century, both rates fell dramatically and in parallel, maintaining the gap. The enormous excess of births over deaths in this period is known as 'the population explosion.' By 1990, the world population was growing at almost 90 million a year.

Surveys show that most women are having more children than they would prefer to have. Further, studies show that the vast majority of women know about various forms of contraception. One World Bank study has shown that family planning programs have little impact unless they are attended by improved living standards and increasing status of women.

Until recently, the world population has been growing faster than exponentially. Although the growth rate has slowed somewhat, there are about 80 million more people each year and about 3 billion more will be added by 2050 (a 50% increase). Population will probably increase more beyond that. Such growth is unprecedented and we cannot predict its long-term effects. The environmental impact of this population increase is bound to be astronomic.

Concerns about low fertility have been present in many countries for at least 100 years. A large population was considered essential to national power. But the issue is never simply a shortage of warm bodies: overall the world population has increased dramatically over this period and untold numbers would immigrate, if allowed. The issue is the number of the 'right sort' of people, defined as those having preferred national, religious, racial, ethnic, or language characteristics. Fertility levels are below replacement in many economically advanced countries.

Census data is often politically influenced and hence inaccurate. The birthrate in developing countries is nearly twice that in developed countries. Most humans live in less developed countries, so the world birthrate is near the higher number. The world birthrate is two and a half times the death rate; we are not close to population stabilization. Almost everywhere, the death rate has been drastically reduced; further changes will not massively affect demographic trends. Changes in fertility rate now control population. Demographic data must be corrected for age structure.

Prior to the Demographic Transition, fertility in northwestern Europe was controlled by limiting marriage. Marriage was regulated by landowners and the churches, and was not allowed unless a man had accumulated the resources necessary to support a family. Long periods of being landless, a servant, or an apprentice, precluded marriage. Once married, there was no control of fertility. But, only about half of adults were married at any given time, so fertility was about half of what it might have been. Eventually, contraception was accepted and fertility within marriage fell.

Prior to Malthus, population growth was seen as good for the power and wealth of a country. The rapid population growth of America was crucial in expelling England (via the Revolution) and France (via the Louisiana Purchase) from the US. But in fact, the numbers of the poor were growing in Europe in the 1700s. Malthus argued that poverty was due to an imbalance between people and resources; since population could rise very fast, it could always outstrip any gains in productivity. He did not anticipate an exponential increase in production or a voluntary decrease in fertility.

European populations grew only slowly during the period 1200-1700; factors include disease and wars. Human feces and rotting animal remains were not sequestered and often contaminated drinking water. Cities were so filthy that more people died in them than were born. About a third of children died in infancy, many from abandonment and lack of care during wet-nursing. Children that survived were subjected to harsh discipline to control their tendency to sin. Ineffective and even harmful treatments, like blood-letting, were all that medicine could offer.

In many regions, the central cultural idea is that of a lineage, a family and its line of male ancestors and descendants. The prime duty in these cultures is to keep the lineage going. Religion is small scale with the ancestors performing many of the functions of gods. Denser populations and larger political entities lead to large-scale religion where conformity is stressed and cultural rules are codified in a book and not subject to discussion with the ancestors. In pre-modern Sub-Saharan Africa, land was not limiting, so a maximum number of children was desired.

In addition to cultural controls acting to maximize fertility, there are important, and often competing, interests of individual families to limit fertility. Unwanted births are dealt with by infanticide in many cultures. Additionally, fertility is regularly controlled by limiting marriage within a culture. Another very important factor in population growth, especially in the tropics, is food availability. Heavy rains in the tropics wash nutrients away, leaving deficient soils. Much of Africa is either too dry or too wet. Africa was, until recently, not densely populated.