Research methods in both behavioral genetics and personality are currently at a crossroads. This chapter examines the disagreement about the genetics of behavior by reformulating its methodological foundation of twin and family studies. It applies the reformulation of older methods to gain realistic understanding of the newer ones that capitalize on the availability of measured DNA. The chapter highlights a particularly problematic aspect of scientific inference in the human behavioral sciences: the inference of causality from nonexperimental data. Religiosity was measured using four items (rated on four-point or five-point ordinal scale) assessing importance of religion, frequency of prayer, attendance at religious services, and attendance at youth groups. Random effects model was estimated in monozygotic (MZ) twin pairs using PROC MIXED in SAS. Linkage analysis has been the earliest molecular method to be adopted in the study of behavior because it requires minimal knowledge of actual genetic sequence.