Social scientific research assumes that religious involvement is primarily, if not exclusively, the product of social-environmental influences There is growing evidence, however, that genetic or other biological factors also play a role Analyzing twin sibling data from the National Survey of Midlife Development in the United States (MIDUS), this study addresses this issue by showing that individual-level variation on four different aspects of religious life—organizational involvement, personal religiosity and spirituality, conservative ideologies, and transformations and commitments—is indeed the product of both genetic and environmental influences Specifically, genetic factors explain 19–65 percent of the variation, while environmental influences account for the remaining 35–81 percent depending upon the aspect of religion under investigation Research of this type enhances contemporary social science by providing a new perspective that nicely supplements existing ones, but it also highlights potential implications, including explanatory power deficiencies and potentially bias