Gottfredson and Hirschi’s 1990 theory has become one of the most tested and empirically supported criminological theories. Even so, much remains unknown about the causes of self-control and the stability of self-control. This study estimated the proportion of variance in self-control that was attributable to genetic factors, shared environmental factors, and nonshared environmental factors. The results of the model-fitting statistics revealed that genetic effects accounted for 64 percent of the variance in the Wave 1 low self-control scale and for 53 percent of the variance in the Wave 2 low self-control scale. The remaining variance was attributable to nonshared environmental influences. The shared environment did not explain any variance in any of the two self-control scales. The stability of self-control during adolescence was also examined. The results revealed some support in favor of Gottfredson and Hirschi’s (1990) stability postulate; the bivariate correlation between the Wave 1 and Wave 2 low self-control scales was r = .64. This suggests that levels of self-control remain relatively stable over the life course. Data for this study came from the National Longitudinal Study of Adolescent Health, which administered a self-report to 90,000 American adolescents in grades 7 through 12 attending 132 middle and high schools; followup interviews were conducted with a random sample of 20,745 adolescents and 17,700 of their primary caregivers in their home. Figures, table, note, and references