This document summarizes 5 days of content in a module about why people cheat. Day 1 compares the sexual attitudes of men and women, finding that men generally prefer more partners and are more approving of adultery. Day 2 examines how differences in male and female reproductive strategies can create conflict. Day 3 explores humanity's natural mating systems and their flexibility. Day 4 analyzes gender differences in rates of and reasons for adultery. Day 5 discusses how modern technology enables virtual forms of non-monogamy and their effects on relationships.