Results revealed substantial support for the hypothesis; the 5HTTLPR polymorphism would not have a statistically significant additive effect on levels of self-control. Across three measures of self-control which spanned approximately 7 years of human development, the 5HTTLPR polymorphism did not have a main effect on low self control. In the non-additive models that tested for gene-environment interactions between 5HTTLPR and delinquent peers, a very different set of results surfaced. These models revealed that 5HTTLPR interacted with delinquent peers to predict a significant amount of variation in measures of self-control assessed during adolescence and early adulthood. Findings highlight the importance of studying both genetic and environmental influences on antisocial phenotypes. Data were collected from the National Longitudinal Study of Adolescent Health, a three wave nationally representative sample of American youth. A total of 20,745 adolescents and 17,700 of their primary caregivers participated in wave 1; a total of 14,738 adolescents completed wave 2 survey instruments; and finally, 15,197 respondents were successfully interviewed at wave 3. Tables, appendix, and references