This article describes biological influences on criminality. It presents illustrative data suggesting a biological sex difference in criminality and heritable differences in this trait among individuals. Next, methods of isolating environmental influences are described, including shared environmental effects, genex environment interactions, and nonshared environmental effects. Using research designs that include just one level of genetic relatedness (e.g., full siblings) is scientifically a poor choice. Using environment-friendly behavior genetic research designs is both proper and would offer many avenues for environmental analysis.