The world population is still increasing and has now reached more than 5.9 billion, according to the most recent United Nations (U.N.) estimates [1].However, the global annual increment -- that is, the number of people added to the world's population each year -- is thought to have peaked between 1985 and 1990 at about 87 million per year. Estimates for 1990-95 are that 81 million persons were added to the population each year [2]. Thanks to long life expectancy and low fertility rates, the populations of most developed countries are now stable or even in decline. Developing countries, however, have not yet achieved that goal. Indeed, the youthful age structure of most developing countries means that their absolute populations continue to grow, even where the rate of increase has declined significantly. (See Global Population Continues to Rise.)

Some regions are therefore closer than others to completing the demographic transition -- the point at which death rates and birth rates are approximately equal and population growth levels off. (See Stabilization Remains a Challenge.) Based on current demographic trends, the world population is projected to reach 9.4 billion by 2050 [3].

Estimates of future population size, however, are highly uncertain. The U.N. projections for 2050 range from a low of 7.7 billion to a high of 11.2 billion. (See Different Assumptions, Different Projections.) The latest "medium-variant" U.N. projection of 9.37 billion is nearly 500 million (4.7 percent) lower than the 9.83 billion medium variant projected in 1994 [4].

The medium-variant projection depends on a number of important assumptions:

• Fertility rates will continue to decline.
• Life expectancy will continue to increase.
• Developing countries will broadly follow the demographic transition already experienced in the industrialized world.

Fertility rates. The most rapid fertility declines have so far occurred in countries that have achieved major improvements in child survival rates and educational levels and have implemented family planning programs (for example, Colombia and Kenya). These developments, in turn, are often associated with economic growth and social changes including improved reproductive rights, rural-urban shifts, new family structures, and new employment patterns, especially changes in female labor force participation rates. Striking fertility declines are evident in rapidly developing countries such as Bangladesh, the Republic of Korea, and Singapore in Asia, and Colombia in Latin America [5]. (See Fertility Declines, Real and Projected.)

Today, the total fertility rate in developed countries is 1.6 children per woman; however, it is still 3.1 children per woman in developing countries. To realize the U.N.'s medium-variant population projection, the world total fertility rate must be lowered from 2.8 (the current rate) to 2.1 children per woman (replacement level) by 2050 [6]. Clearly, Africa is the region in which the future trend in fertility is significantly different from that of the past, and for this reason, its projection must be considered the most speculative.

Thailand has experienced one of the strongest rates of economic development -- and one of the steepest declines in fertility -- in the world. Thailand's total fertility rate declined from 6.59 children per woman during 1955-60 to 1.94 during 1990-95 [7]. In 1960, the government launched a vigorous family planning program and made contraceptives freely available all over the country. As a result, the population growth rate has fallen from a high of 3.1 percent to its current level of 0.9 percent. In poorer countries, duplicating this kind of experience would be a formidable challenge. Even so, population programs and social change are beginning to reduce fertility rates in many of the least developed countries as well. For example, Tanzania experienced a small but significant decline in total fertility rate, which dropped from 6.8 children per woman in the 1960s and 1970s to just under 6 children per woman in 1990-95 [8]. The continued support of donor countries and institutions for the country's family planning program, as well as economic and educational development, are likely to be critical in maintaining progress. The government of Tanzania has estimated that, if it is to meet its target of universal access to family planning services by 2015, it must provide for 5.6 million contraceptive users by 2015, almost a sixfold increase over the 1995 demand [9].

Life expectancy. Low death rates primarily reflect improved child survival rates, and child survival is a key determinant in family size. Developing countries have halved the life expectancy gap with industrialized countries over the past 40 years. The least developed countries, however, have experienced smaller gains, and life expectancy in these countries lags at about 50 years. In the medium-variant projection, life expectancy rates are assumed to rise to 81, 76, and 72 years in developed, developing, and least developed countries, respectively, by 2050. This increase presupposes, among other things, major progress in the fight against infectious diseases [10]. (See Linking Environment and Health.)

References and notes

1. United Nations (U.N.) Population Division, World Population Prospects 1950-2050 (The 1996 Revision), on diskette (U.N., New York, 1996).

2. Ibid.

3. Op. cit. 1.

4. United Nations (U.N.) Population Division, World Population Prospects 1950-2050: The 1996 Revision, Annex 1: Demographic Indicators (U.N., New York, 1997), pp. 11-45.

5. Ibid., p. 121.

6. Op. cit. 4, pp. 124-125.

7. Op. cit. 1.

8. Op. cit. 4, Table A.18, pp. 120-122.

9. United Nations Fund for Population Activities (UNFPA), Country Profile: United Republic of Tanzania (UNFPA, New York, 1992).

10. Op. cit. 4, pp. 164, 179.