This chapter explores the practical significance of the approach to natural law presented in the previous chapter. I examine various types of scientifically based arguments about what evolution implies, or does not imply, concerning sex, marriage, and family, and investigate whether and how evolved capacities and needs are captured in institutions of marriage and the family and in moral norms governing sexual behavior. This cluster of topics provides a particularly interesting context for investigating whether there is an evolutionary basis for the moral norms advanced by the natural-law tradition of Christian ethics, and whether there might be evolutionary support for modifying these norms in some ways. In evolutionary terms, we can ask whether certain moral beliefs have supported acts or practices that are more adaptive than their alternatives. This involves descriptive as well as properly normative concerns. Identifying certain acts or practices as more in keeping with our “inclusive fitness” does not in and of itself provide sufficient moral justification for acts and practices under consideration.

The chapter has three sections: the first concerns evolutionary theory, the second offers a natural-law evaluation of evolutionary views of sex, and the third considers some aspects of normative ethics in the domain of sex, marriage, and family. My thesis is that contemporary natural-law ethics ought to attempt a critical and selective appropriation of evolutionary views of sex, marriage, and the family in light of its inclusive vision of the human good.