The concept of natural fertility was first defined by the French demographer Louis Henry (1911–1991) in the 1953 study, "Fondements théoriques des mesures de la fécondité naturelle," to refer to the fertility of a population not practicing any form of birth control. In 1961, he gave a more precise definition in two papers. In "Some Data on Natural Fertility," published in Eugenics Quarterly, and frequently quoted, Henry stated clearly that he preferred the qualifier natural to physiological or biological because "social factors may also play a part–sexual taboos, for example, during lactation" (Henry 1961, p. 81). He also suggested a simple means for separating controlled fertility from natural fertility: "control can be said to exist when the behavior of a couple [at a given age or marriage duration] is bound to the number of children already born and is modified when this number reaches the maximum which the couple does not want to exceed" (p. 81).
The role of marriage–a typical non-biological variable–was implicit in this definition because Henry was in fact interested in marital fertility: Out-of-wedlock births were rare in the France of the ancien régime (the fifteenth through eighteenth centuries). Even with this restriction, the levels of natural fertility that he found exhibited a wide range, varying by a factor of two. The highest recorded total fertility in a population is around ten children per woman; but in some parts of seventeeth-century France fertility–also taken to be natural–was below five children per woman. This variation is due to both behavioral factors (age of marriage, sexual behavior, duration of breastfeeding) and biological factors (fecundability, the post-partum non-susceptible period, the rate of fetal wastage–which vary both among individuals and among populations). It has been estimated that a woman who is continuously in a sexual union between the ages of 15 and 50 years, not breastfeeding her children, and not practicing any form of birth control, would bear 15 children on average.
Although age-specific marital fertility rates may vary among natural-fertility populations, the age profile of these rates is characteristic. This property has been used to develop models of fertility (for example, the Coale-Trussell model) which, fitted to the observed fertility schedule from any given population, can give an indication of how far the observed rates are from the standard profile, and thus how far fertility in that population is from a natural regime. The widely-used decomposition of fertility into its proximate determinants, the framework developed by John Bongaarts, also owes much to the scheme developed by Henry.
The concept of natural fertility is explicitly based on the absence of attempts by couples to limit the number of their children, not on the absence of efforts to space them. Strictly speaking, spacing behavior that was independent of the number of children born could not be detected. It is however, very unlikely that a population would develop an effective form of spacing behavior that was independent of any limiting intentions. Breastfeeding could be construed to be such a practice. However, its primary aim is to help the child survive and grow; it is impossible to separate this purpose from a possible attempt tempt by the mother to space her births as a way of controlling their final number.
Coale, Ansley J., and James Trussell. 1974. "Model Fertility Schedules: Variations in the Age-Structure of Childbearing in Human Populations." Population Index 40(2): 185–258.
Henry, Louis. 1953. "Fondements théoriques des mesures de la fécondité naturelle." Revue IIS 21(3): 135–151.
——. 1961. "Some Data on Natural Fertility." Eugenics Quarterly 8(2): 81–91.
Leridon, Henri. 1989. "Fécondité naturelle et espacement des naissances." Annales de démographie historique 1988, pp. 21–34.
Leridon, Henri, and Jane Menken, eds. 1979. Natural Fertility/Fécondité Naturelle. Liège, Belgium: Ordina.