Population dynamics are one of the key factors to consider when thinking about development. In the past 50 years the world has experienced an unprecedented increase in population growth (Figure 3.1). Do you know why?

A "natural population increase" occurs when the birth rate is higher than the death rate. While a country's population growth rate depends on the natural increase and on migration, world population growth is determined exclusively by the natural increase.

Around the world, death rates gradually decreased in the late 19th and the 20th centuries, with death rates in the developing world plummeting after World War II thanks to the spread of modern medicine. In much of the developing world the decline in death rates preceded the decline in birth rates by 20 years or more, resulting in record- high rates of population growth of 3 percent or even 4 percent a year. Since the 1960s birth rates have also been declining rapidly in most developing countries except those in Sub-Saharan Africa and the Middle East. This trend in birth rates in the developing world is comparable to what took place in Europe and the United States in the 19th century (Figure 3.2).

Today's low-income countries still have the world's highest birth rates (see Map 3.1), although women tend to have fewer children than before. The reasons for lower fertility are varied, but most are related to developing countries' economic growth and human development (Figure 3.3; see also Chapters 4 and 7). Parents choose to have smaller families when health conditions improve so that they no longer have to fear that many of their babies might die, and when they do not have to rely on their children to work on the family farm or business or to take care of them in their old age. In addition, more parents are sending their daughters to school, which is important also because women with basic education tend to produce healthier children and smaller families. More women now have opportunities to work outside the home, so they are starting their families later and having fewer children. On top of all that, access to family planning is improving, so parents can control the number and spacing of their children.

A lower fertility rate may not immediately lead to a lower birth rate and lower population growth if a country has a larger number of men and women in their reproductive years than before. Population growth caused by more women giving birth even though each has the same number of or fewer children is called "demographic momentum." Demographic momentum is particularly significant in developing countries that had the highest fertility rates 20-30 years ago.

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