In non-human mammals, the positive relationship between resources or dominance and fertility is well established and relatively simple. When people have manipulated the resource-fertility relationship through cultural practices, these normal relationships have sometimes been subverted. The sex difference in remarriage rates undoubtedly contributes to the greater fertility of men's second marriages, compared to women's, even in societies with late ages at first marriage and socially imposed monogamy. In fact, Birdsall's within-society comparisons show a linear positive relationship between wealth and fertility in India. A non-linear, roughly bell-shaped curve leads to suspect class or wealth differences in investment pattern superimposed on the standard functional relationships, as families compete in different environments. Sex differences in wealth-fertility relationships need further exploration. Through the demographic transitions, certain predictable ecological rules underlie patterns of fertility, mortality, and migration, although these may be constrained by a variety of cultural complexities and interactions.