We have found that the !Kung population, in comparison to others, has a scarcity in the supply of potential mothers. Reproductive opportunities are lost in primary sterility, before fecundity is established, and secondary sterility, following childbearing but prior to the usual age of menopause. In this chapter we will explore the evidence for the various determinants of primary and secondary sterility for the women in the childbearing years of life during the 1963–1973 period of observation.