Gottfredson and Hirschi's theory contained two propositions that have been the source of an emerging line of empirical scrutiny. First, according to the general theory of crime, levels of self-control are largely determined by parental management techniques and not by biogenic factors. Second, Gottfredson and Hirschi argued that low self-control should remain relatively stable over the life course. Data from twins drawn from the National Longitudinal Study of Adolescent Health were used to test these two hypotheses. The results of univariate model-fitting techniques revealed that genetic factors accounted for between 52 and 64 percent of the variance in low self-control, with the remaining variance attributable to the nonshared environment. Further, low self-control was stable over a two-year time period (r = .64). Bivariate Cholesky decomposition models indicated that the stability of self-control was determined almost exclusively by genetic factors, and that genetic factors also explained a moderate amount of change in self-control.