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Developing countries accounted for 97 percent of this growth because of the dual effects of high birth rates and young populations (see figure). Conversely, in the developed countries the annual number of births barely exceeds deaths because of low birth rates and much older populations. By 2025, it is likely that deaths will exceed births in the developed countries, the first time this will have happened in history.

Nearly All Future Population Growth Will Be in the World’s Less Developed Countries.

Source: United Nations Population Division, World Population Prospects: The 2010 Revision, medium variant (2011).

While virtually all future population growth will be in developing countries, the poorest of these countries will see the greatest percentage increase. As defined by the United Nations, these 48 countries have especially low incomes, high economic vulnerability, and poor human development indicators such as low life expectancy at birth, very low per capita income, and low levels of education. Of these countries, 33 are in sub-Saharan Africa, such as Burundi, Ethiopia, Mozambique, and Zambia; 14 in Asia, including Bangladesh, Cambodia, Nepal, and Yemen; and one in the Caribbean, Haiti. They are growing at 2.4 percent per year and are projected to reach at least 2 billion by 2050.

Trends in Developing Countries

Africa. By far, the largest regional percentage increase in population by 2050 will be in Africa, whose population can be expected to at least double from 1.1 billion to about 2.3 billion. That projection, however, depends on the assumption that sub-Saharan Africa’s total fertility rate (TFR, the average number of children per woman) will decline from 5.1 to approximately 3.0 by 2050. That decline, in turn, assumes that the use of family planning in the region will rise significantly. But recent surveys from many sub-Saharan African countries have indicated that TFR decline is either slower than projected or is not taking place at all. Only 20 percent of married women in sub-Saharan Africa use a modern form of family planning, the lowest rate in the world.

Asia. With a current population of 4.3 billion, Asia will likely experience a much smaller proportional increase than Africa but will still add about 1 billion people by 2050. Much of Asia’s future population growth will be determined by what happens in China and India, two countries that account for about 60 percent of the region’s population. In India, the largest unknowns are future fertility trends in the heavily populated northern states where TFRs of about 3.5 are well above those of India’s southern states. Asia’s TFR is 2.2 (2.5 when the large statistical effect of China is removed). Excluding China, 47 percent of women in Asia use a modern form of con’raception. Within Asia, several of the more economically advanced countries such as Japan, Singapore, South Korea, and Taiwan have TFRs of 1.4 or even lower. In Japan, 24 percent of the population is already ages 65 and older, a proportion certain to continue growing. Thus far in Japan, government efforts to restore somewhat higher past levels of the TFR have not been successful.

Latin America. Latin America and the Caribbean is the developing region with the smallest proportional growth expected by 2050, from 599 million to 740 million, largely due to fertility declines in several of its largest countries such as Brazil and Mexico. The region’s TFR is currently about 2.2 children per woman, and the use of modern contraception, at 67 percent, rivals that of developed countries.

Trends in Developed Countries

The very sharp decline in fertility in the developed countries, and how long it has lasted, has been completely unforeseen. TFRs of 1.4, 1.3, and even lower, took demographers by surprise. Yet not all developed countries tell the same story. In countries such as France and Norway, social programs to support families—such as generous maternity leave and subsidies for child care—have kept TFRs close to 2.0.

Europe is likely to be the first region in history to see long-term population decline largely as a result of low fertility in Eastern Europe and Russia. Europe’s population is projected to decrease from 740 million to 732 million by 2050. The population of the 27 countries in the European Union, around 502 million, should roughly maintain their current size, even with large increases in the elderly population compared with younger age groups. The recent global recession has dimmed hopes in many European countries on the prospects of raising low birth rates to mitigate the economic effects of unprecedented proportions of the elderly, such as shortages in pension systems and rising health care costs for the “old-old” (ages 85 and above). In Australia, Canada, New Zealand, and the United States, continued growth from higher births or continued immigration, or both, are expected, although these countries have not been immune to lower birth rates due to the recession. In the United States, for example, the TFR was 1.9 births per woman in 2010.

The Demographic Divide

The radically different demographic situation between developed and developing countries illustrates the “demographic divide”: the vast gulf in birth and death rates among the world’s countries. On one side of this divide are mostly poor countries with relatively high birth rates and low life expectancies. On the other side are mostly wealthy countries with birth rates so low that population decline is all but guaranteed and where average life expectancy extends past age 75, creating rapidly aging populations.

The table shows just how wide these gaps have become.

Even though Tanzania and Spain have almost the same population size today, Tanzania is projected to more than double its population from 48 million to 138 million in 2050. Spain’s population will only slightly increase, from 46 million today to 48 million by 2050. The cause of this enormous difference is lifetime births per woman. Tanzania’s total fertility rate of 5.4 children per woman is almost four times greater than Spain’s rate of 1.4.

Carl Haub is a senior demographer at the Population Reference Bureau, and co-author of PRB’s 2012 World Population Data Sheet.