The study revealed three broad findings. First, poor language skills were predictive of low self-control, both cross-sectionally and longitudinally. Second, the covariation between language skills and self-control was linked to both genetic and environmental factors. Third, after controlling for genetic influences on self-control, language skills were predictive of variation in individual levels of self control. Given these findings and those reported in previous studies, Gottfredson and Hirsch’s (1990) theory on the development of self-control should be revised in at least two ways. First, the belief that biological and genetic factors have no effect on self-control conflict with a persuasive line of behavioral genetic research that shows otherwise. Second, Gottfredson and Hirschi’s theory should emphasize the important role that the brain, especially the prefrontal cortex plays in structuring levels of self-control. This study used data from the Early Childhood Longitudinal Survey, Kindergarten Class of 1998-1999 (ECLS-K), a nationally representative and longitudinal sample of children that consists of more than 20,000 children. The ECLS-K also oversampled twins. Overall, 310 twin pairs were identified and included in the ECLS-K, which were selected for inclusion in the current study. Self-control was measured with an adapted version of Gresham and Elliott’s (1990) Social Skills Rating Scale, which was included in the ECLS-K. During wave 1 interviews, children completed a direct cognitive assessment that measured their language skills on three different dimensions. The analysis of the data from these instruments is described in this article. 6 tables, 59 references, and appended basic univariate variance components model for the language skills scale and for the low self-control scales