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TABLE 1. Combinations of the probability of surviving to age 28 [p(28)] and total fertility rate (TFR) that lead to a specified growth rate (r) and corresponding net reproduction rate (NRR). Boldface numbers correspond to life expectancy of at least 50 years
|Probability of surviving to age 28 that produces given NRR when:|
|(%)||NRR||TFR 2||TFR 3||TFR 4||TFR 5||TFR 6||TFR 7||TFR 8||TFR 9|
TFR = total fertility rate = average number of
children born to a woman who lives to age 50.
NRR = net reproduction rate = average number of daughters a girl born today will contribute to the next
generation ~ e28r/100 ~ TFR (.488) p(28).
.488 ~ proportion of babies that are girls.
p(28) = probability a girl will survive to age 28 (the mean age of the fertility schedule); see table 2 for relationship between p(28) and life expectancy.
a. Because the calculated value for this cell was greater than one and, therefore, outside the possible range, it was replaced by the maximum possible value.
TABLE 2. Relationship between expectation of life at birth and proportion of women surviving to exact age 28 [p(28)]
|Life expectancy (yr)||p(28)|
Table 3 is included here to give some concrete background to our discussion. The data are for 33 countries in which Demographic and Health Surveys were taken between 1985 and 1992. The information on life expectancies and growth rates comes from the Population Reference Bureau  and that on TFRs from Muhuri et al. . Nearly all (30 of the 33) of these countries have life expectancies over SO, and only 5 have lowered their growth rates to under 2% per year. The TFRs are for the five years prior to the survey date and are highly variable.
In addition to showing the great variation in overall fertility, the table shows the general trend of declining fertility with increased socio-economic status, here measured by education of the woman. For most countries with low fertility, that level has been achieved through the use of methods of fertility control, although for some parts of Africa, unusually high rates of sterility are believed to be important determinants of reproduction. When combined with the life expectancies also shown in table 3, these fertility rates lead to the substantial growth rates seen there. By contrast, the countries of the West all have growth rates under 1.5%.
Populations that offer evidence of few or no practices intended to limit the number of children a woman bears over the course of her lifetime have reproductive patterns that have been termed natural fertility. Wood  reports that 70 such populations have been identified; their average total fertility was 6.1 children per woman, and the standard deviation was 1.2. Thus, the variation in fertility, even without deliberate control, is substantial. The great French demographer Louis Henry earlier estimated total marital fertility for nine populations that did not limit fertility. If all women married at age 15 and survived the reproductive years, the average number of children per woman would vary from 8.5 to 14.1 . Yet there is substantial consistency. Henry noted that there is a common age pattern to the fertility of married women in such populations; fertility remains high until the mid-thirties and then declines in a fashion that produces a concave curve. This curve was later codified in the Coale and Trussell  set of schedules of fertility, in which all populations with natural fertility were described by a standard schedule that represented the age pattern and a multiplier, a parameter that represented the relative level of fertility in a particular population compared with a standard reference population. In other words, if marital fertility in a particular population was about 90% of the reference at age 20, they found it was about 90% as high at all other ages. They went further and found that populations that control their fertility have age patterns that could be described well with the addition to their equation of a second parameter that represents the degree of fertility control. Their work in developing this model of fertility and demonstrating that it fits a wide range of populations well permits us to conclude that although there is a great deal of variation to be explained in worldwide patterns of reproduction, in populations that do not practice deliberate methods of control, there are common patterns that are of great interest.
TABLE 3. Life expectancy (e00), growth rate (r), and total fertility rate (TFR) for all women aged 15 44 according to education in 33 countries in which there were demographic and health surveys between 1985 and 1992
|All women||No schooling||Did not complete primary school||Completed primary school||Secondary school or higher|
|Near East and North Africa|
|Trinidad and Tobago||67||1.4||3.14||(2.28)||3.67||3.69||2.95|
Sources: e00 and r from ref. 6; TFR from ref. 7.
* <50 women-years of exposure in at least one or the age groups. Numbers in parentheses: soots women-years of exposure in at least one of the age groups.
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