Fertility. Fertility is the natural capability to produce offspring. As a measure, fertility rate is the number of offspring born per mating pair, individual or population. Fertility differs from fecundity, which is defined as the potential for reproduction A lack of fertility is infertility while a lack of fecundity would be called sterility. Human fertility depends on factors of nutrition, sexual behavior, consanguinity, culture, instinct, endocrinology, timing, economics, way of life, and emotions. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity. While fertility can be measured, fecundity cannot be. Demographers measure the fertility rate in a variety of ways, which can be broadly broken into period measures and cohort measures. Period measures refer to a cross-section of the population in one year. Cohort data on the other hand, follows the same people over a period of decades. Both period and cohort measures are widely used. Crude birth rate-the number of live births in a given year per 1,000 people alive at the middle of that year. One disadvantage of this indicator is that it is influenced by the age structure of the population. General fertility rate-the number of births in a year divided by the number of women aged 15-44, times 1000. It focuses on the potential mothers only, and takes the age distribution into account. Child-Woman Ratio-the ratio of the number of children under 5 to the number of women 15-49, times 1000. It is especially useful in historical data as it does not require counting births. This measure is actually a hybrid, because it involves deaths as well as births. Coale's Index of Fertility-a special device used in historical research. Total fertility rate-the total number of children a woman would bear during her lifetime if she were to experience the prevailing age-specific fertility rates of women. TFR equals the sum for all age groups of 5 times each ASFR rate. Gross Reproduction Rate-the number of girl babies a synthetic cohort will have. It assumes that all of the baby girls will grow up and live to at least age 50. Net Reproduction Rate-the NRR starts with the GRR and adds the realistic assumption that some of the women will die before age 49; therefore they will not be alive to bear some of the potential babies that were counted in the GRR. NRR is always lower than GRR, but in countries where mortality is very low, almost all the baby girls grow up to be potential mothers, and the NRR is practically the same as GRR. In countries with high mortality, NRR can be as low as 70% of GRR. When NRR = 1.0, each generation of 1000 baby girls grows up and gives birth to exactly 1000 girls. When NRR is less than one, each generation is smaller than the previous one. When NRR is greater than 1 each generation is larger than the one before. NRR is a measure of the long-term future potential for growth, but it usually is different from the current population growth rate. Main article: fertility factor A parent's number of children strongly correlates with the number of children that each person in the next generation will eventually have. Factors generally associated with increased fertility include religiosity, intention to have children, and maternal support. Factors generally associated with decreased fertility include wealth, education, female labor participation, urban residence, cost of housing, intelligence, increased female age and increased male age. The Three-step Analysis of the fertility process was introduced by Kingsley Davis and Judith Blake in 1956 and makes use of three proximate determinants: The economic analysis of fertility is part of household economics, a field that has grown out of the New Home Economics. Influential economic analyses of fertility include Becker, Mincer, and Easterlin. The latter developed the Easterlin hypothesis to account for the Baby Boom. Bongaarts proposed a model where the total fertility rate of a population can be calculated from four proximate determinants and the total fecundity. The index of marriage, the index of contraception, the index of induced abortion and the index of postpartum infecundability. These indices range from 0 to 1.