The central chapters of this book focus on the development and growth of insular origin legends over time by studying a key subset of themes that came to take on particular significance within this corpus. Tracing the expansion and increasing centrality of these themes over time allows us to witness the influence that individual texts within the corpus of material containing early insular origin legends had on the development of these legends themselves. Chapter Four focuses on intermarriage and incest, first tracing medieval understanding of these concepts in biblical tradition before examining contemporary evidence for intermarriage and incest in early medieval legal and historical texts. Origin legends expanded further to envision what might have happened after a foundational ancestor who emigrated alone or with a small band of followers arrived to the insular region. Women were necessary in order to preserve the population, and there were really only two solutions to the problem: intermarriage or incest. This chapter examines each of these motifs in turn, studying how they were introduced into insular origin legends and expanded to create different narrative possibilities for political commentary as the corpus grew.