Demographers count fertility in half a dozen ways, and each one hides something
To a demographer, fertility means babies actually born, not the ability to have them, which is called fecundity and cannot be measured directly. So population scientists juggle a toolkit of ratios, from the crude birth rate to the net reproduction rate, each correcting a blind spot in the one before.
The simplest yardstick, the crude birth rate, counts live births in a year per 1,000 people alive at midyear, but a population with many elderly people will look less fertile than it really is. The general fertility rate fixes that by dividing births only by women aged 15 to 44. For historical records without birth registers, the child-woman ratio compares children under five with women aged 15 to 49, a hybrid measure because infant and adult deaths both leak into it.
The best-known figure, the total fertility rate, estimates how many children a woman would bear if she lived through today's age-specific birth rates. The gross reproduction rate counts only daughters and assumes all of them survive to 50. The net reproduction rate adds realistic deaths; in high-mortality countries it can fall to 70 percent of the gross figure. When it equals exactly one, each generation of 1,000 girls produces 1,000 girls, and the population replaces itself.
Demographers also separate period measures, which slice one year across the whole population, from cohort measures, which follow the same people for decades. Kingsley Davis and Judith Blake introduced a three-step analysis of the fertility process in 1956, and John Bongaarts later built a model deriving the total fertility rate from four indices for marriage, contraception, induced abortion and postpartum infecundability, each ranging from 0 to 1. Economists joined in: Richard Easterlin used his work to explain the Baby Boom.
Studies link higher fertility with religiosity, the intention to have children and maternal support, and lower fertility with wealth, education, women's work outside the home, city living and housing costs. Nutrition matters as well: the demographer Ruth Frisch estimated the energy cost of producing one baby at 50,000 calories.
Source: Fertility
