To address concerns that these gene-by-environment effects were due to gene-environment correlation (polygenic scores predicted 1% and 2% of the variance in parental monitoring and peer substance use), we re-ran our gene-by-environment analyses using residualized polygenic score, parental monitoring, and peer substance use variables (e.g., by saving the residuals from a regression of polygenic scores onto parental monitoring and vice versa). Using residualized variables statistically eliminates gene-environment correlation in the model because the genetic and environmental effects have been partialled from one another. The gene-by-environment effects continued to be significant [Bparents × polygene = -1.60, t(154) = -1.88, p = 0.02; Bpeers × polygene = 1.97, t(155) = 2.84, p < .01], and again indicated that genetic variance increased at low levels of parental monitoring and high levels of peer substance use.
