It is a commonplace observation that fertility is high in the so-called underdeveloped societies. Sometimes this leads to the erroneous view that their fertility is limited only by what is biologically possible so that birth rates reach a mythologically high natural level. In this idealized view free and unrestrained sexual unions beginning at an early age produce very high fertility as an incidental result without deliberate intent and without much individual or social concern about family size. A more plausible general proposition is that reproduction, whether at high or low levels, is so important to the family and to society everywhere that its level is more or less controlled by cultural norms about family size and such related matters as marriage, timing of intercourse, and abortion. In each society the cultural norms about these vital matters are consistent with social institutions in which they are deeply embedded. Changes in fertility are unlikely without prior or, at least, simultaneous changes in these institutions.