The full force of social controls on the speed of human reproduction can be clearly appreciated when we contrast reproductive potential with realized levels of fertility. Although the calculation of potential fertility, or fecundity, is beset with conceptual, technical and measurement problems, a consensus has emerged that for most human populations, the upper limit on the average number of live births per woman must lie between 13 and 17 (Bongaarts & Potter, 1983: p. 79). This is not to deny that some individual women can and do produce many more than this number, but for populations rather than individuals in which biology is the only factor restricting fertility, an acceptable average number of births per woman is around 15.3. Even this number of births can only be achieved by a rare combination of circumstances. We can list these very briefly:
– continuous exposure to the risk of conception between menopause and menarche, meaning a steady sexual relationship throughout the woman's fertile life;
– complete avoidance of any contraceptive method, including ‘natural’ methods, during the entire reproductive lifespan;
– no use of induced abortion or of any activity which might be undertaken to provoke a miscarriage;
– avoidance of any breastfeeding practice.
Clearly, this combination of conditions is very rare, helping to explain why measured levels of ‘natural’ marital fertility rarely surpass 11 or 12 live births. Some historical data based on written records or family reconstitution methods have thrown up a few populations in which most of the above conditions are met (see the summary table in Leridon, 1977: pp. 107–9).