World Development Report 1984

mary measures are weighted by population in has declined to replacement level because past
1970. high growth rates will have produced an age distri-
The estimates of population for mid-1982 are pri- bution with a relatively high proportion of women
marily based on data from the UN Population Divi- in, or still to enter, the reproductive ages. Consesion.
In many cases the data take into account the quently, the birth rate will remain higher than the
results of recent population censuses. death rate and the growth rate will remain positive
The projections of population for 1990 and 2000, for several decades. A population takes 50-75
and to the year in which it will eventually become years, depending on the initial conditions, before
stationary, were made for each economy sepa- its age distribution fully adjusts to the changed
rately. Starting with information on total popula- fertility rates.
tion by age and sex, fertility rates, mortality rates, To make the projections, assumptions about
and international migration rates in the base year future mortality rates were made in terms of
1980, these parameters were projected at five-year female life expectancy at birth (that is, the number
intervals on the basis of generalized assumptions of years a newborn girl would live if subject to the
until the population became stationary. The base- mortality risks prevailing for the cross-section of
year estimates are from updated computer print- population at the time of her birth). Economies
outs of the UN World Population Prospects as Assessed were first divided according to whether their priin
1982, from the most recent issues of the UN mary-school enrollment ratio for females was
Population and Vital Statistics Report and International above or below 70 percent. In each group a set of
Migration: Levels and Trends, and from the World annual increments in female life expectancy was
Bank, the Population Council, the US Bureau of assumed, depending on the female life expectancy
the Census, and recent national censuses. in 1980-85. For a given life expectancy at birth, the
The net reproduction rate (NRR) indicates the annual increments during the projection period are
number of daughters that a newborn girl will bear larger in economies having a higher primaryduring
her lifetime, assuming fixed age-specific school enrollment ratio and a life expectancy of up
fertility rates and a fixed set of mortality rates. The to 62.5 years. At higher life expectancies, the incre-
NRR thus measures the extent to which a cohort of ments are the same.
newborn girls will reproduce themselves under To project the fertility rates, the first step was to
given schedules of fertility and mortality. An NRR estimate the year in which fertility would reach
of 1 indicates that fertility is at replacement level: replacement level. These estimates are speculative
at this rate childbearing women, on the average, and are based on information on trends in crude
bear only enough daughters to replace themselves birth rates (defined in the note for Table 20), total
in the population. fertility rates (also defined in the note for Table 20),
A stationary population is one in which age- and female life expectancy at birth, and the performsex-specific
mortality rates have not changed over ance of family planning programs. For most econoa
long period, while age-specific fertility rates have mies it was assumed that the total fertility rate
simultaneously remained at replacement level would decline between 1980 and the year of reach-
(NRR=1). In such a population, the birth rate is ing a net reproduction rate of 1, after which fertilconstant
and equal to the death rate, the age struc- ity would remain at replacement level. For most
ture also is constant, and the growth rate is zero. countries in sub-Saharan Africa, however, total
Population momentum is the tendency for popula- fertility rates were assumed to remain constant
tion growth to continue beyond the time that until 1990-95 and then to decline until replacement
replacement-level fertility has been achieved; that level was reached; for a few they were assumed to
is, even after NRR has reached unity. The momen- increase until 1990-95 and then to decline. Also for
tum of a population in the year t is measured as a a few countries in Asia and the Middle East, the
ratio of the ultimate stationary population to the rates were assumed to remain constant for some
population in the year t, given the assumption that years before beginning to decline. In several indusfertility
remains at replacement level from the year trial economies, fertility is already below replacet
onward. In India, for example, in 1980 the popu- ment level. Because a population will not remain
lation was 687 million, the ultimate stationary pop- stationary if its net reproduction rate is other than
ulation assuming that NRR = 1 from 1980 onward 1, it was assumed that fertility rates in these econowas
1,195 million, and the population momentum mies would regain replacement levels in order to
was 1.74. make estimates of the stationary population for
A population tends to grow even after fertility them. For the sake of consistency with the other
281