# XXVi INTRODUCTION

the eventual stationary population under the special assumptions that fertility drops to replacement level immediately, mortality stays constant at current levels, aad no migration takes place. (Previous estimates of momentum in _World Development Report_ have allowed mortality to decline.) It is effectively an indicator of the amount of future population growth that can be attributed solely to the current age structure of a population. An illustration of the importance of momentum is the fact that more than one billion of the stationary world population will be added because of momentum after replacement fertility is achieved.

Population momentum is considerably lower for more developed than for less developed countries. For more developed countries, only 10 percent growth can be expected because of the age structure. Nevertheless, this will still be the main source of growth for these countries, as indicated by the stationary population being only 10 percent larger than the current population.

Across continents, population momentum is highest for Africa at 1.6, but is clearly only a small part of future growth. The projected stationary African population is more than five times its 1985 population.

Age structure. Of the 4.84 billion people in 1985, 60 percent were in the working ages of 15-64, giving a dependency ratio (of those younger and older combined per hundred members of this group) of 67. By 2025, the world dependency ratio is projected to fall to 53. Children and youth under 15 will have fallen from 34 to 26 percent of the population, while the aged 65 and over will have risen from 6 to 9 percent (Table 7; see Table 19 for percentages).

Projected trends in more developed and less developed countries are quite different. In more developed countries, the number of children and youth will actually decline from 260 to 248 million, and as a proportion of the population they will fall to only 19 percent. The aged, on the other hand, will almost double, and by 2025 should be almost as numerous as children and youth. Thus, the dependency ratio in more developed countries will rise from 50 to 58.

In less developed countries, children and youth will continue to increase, from 1.4 to 1.9 billion, though as a proportion of the population they will fall from 37 to 27 percent. The aged, who are far fewer, will be increasing much faster, from 153 to 514 million, or from 4 to 7 percent of the population. The dependency ratio will fall from 71 to 52, as the working-age population also grows very fast, at a 2.3 percent annual rate up to 2000 and at a 1.7 percent rate from 2000 to 2025.

These contrasts are summarized in four population pyramids, for more developed and less developed countries for 1985 and 2025 (Figures 11-14). The pyramids for the two groups of countries are drawn to different scales. For more developed countries, growth will be modest and mainly involve the pyramid becoming increasingly rectangular. For less developed countries, all age groups will increase, but, given previous high fertility and high infant and child mortality and expected declines in these parameters, the pyramid will become bulkier in the middle, among working-age adults.

Across continents, Africa stands out with 45 percent of its population--0 points more than any other continent--under 15. Consequently, Africa has a dependency ratio of 94, as opposed to only 57-66 in Asia, America, and Oceania and only 51 in Europe and the U.S.S.R. All three age groups--the young, those of working age, and the aged--will grow considersbly faster between 1985 and 2025 in Africa than anywhere else. The older age groups will grow somewhat faster than the younger age groups in Africa, as will also be true in all other continents. However, by 2025, those under 15 will still be 36 percent of the population of Africa, as opposed to 19-25 percent of the population of other continents.