Although the transmission of religiousness has been assumed to be purely cultural, behavior genetic studies have demonstrated that genetic factors play a role in the individual differences in some religious traits. This article reviews the extant behavior genetic literature and presents new analyses from the “Virginia 30,000” on the causes of variation in religious affiliation, attitudes, and practices, and relates these to personality as construed by Eysenck. Results indicate that religious affiliation is primarily a culturally transmitted phenomenon, whereas religious attitudes and practices are moderately influenced by genetic factors. Further, Eysenck’s personality traits do not mediate genetic influences on religiousness, but significant negative genetic correlations are found between church attendance and liberal sexual attitudes. Implications and possibilities for future studies are discussed.