The publication of a paper the following year (Purcell 2002) that provided straightforward scripts for continuous gene-environment interaction models using the most widely used program for twin analyses, Mx (Neale 2000), led to a surge of papers studying gene-environment interaction in the twin literature. These scripts also offered the advantage of being able to take into account gene-environment correlation in the context of gene-environment interaction. This was an important advance because previous examples of gene-environment interaction in twin models had been limited to environments that showed no evidence of genetic effects so as to avoid the confounding of gene-environment interaction with gene-environment correlation. Using these models, we have demonstrated that genetic influences on adolescent substance use are enhanced in environments with lower parental monitoring (Dick et al. 2007c) and in the presence of substance-using friends (Dick et al. 2007b). Similar effects have been demonstrated for more general externalizing behavior: Genetic influences on antisocial behavior were higher in the presence of delinquent peers (Button et al. 2007) and in environments characterized by high parental negativity (Feinberg et al. 2007), low parental
