Although men are substantially more interested than women in casual sex, there is ample variation in this trait (sociosexuality) within both sexes. One theory hypothesizes that within-sex sociosexual variation results from genetic variation maintained by frequency-dependent selection. If so, sociosexuality should be substantially heritable. A competing theory is that children acquire their mating strategy after observing their parents' relationship. By this theory, sociosexuality should reveal a strong shared environmental component. The authors studied genetic and environmental influences on sociosexuality using a large, representative volunteer twin sample. Parental marital instability was modestly associated with sociosexuality, but this could have been due to either genetic or environmental factors. Consistent with genetic theory, familial resemblance appeared primarily due to additive genetic rather than shared environmental factors.