Next, we tested for association between the GRS and the experience of traumatic life events, neighborhood social cohesion and neighborhood physical disorder. That is, we conducted a test of Spearman's rGE in SAS (SAS Institute, 2003), as the presence of rGE undermines the interpretation of GxE.46 We then examined how the GRS interacted with environmental exposures, for which we did not find a significant rGE to predict cigarette smoking in this population. Additive interactions were tested in SAS (SAS Institute, 2003), using sex, age and ancestry as covariates. We assessed the interaction between the GRS and each environmental variable on the additive scale by including a cross-product term in separate linear-Poisson regression models. The estimate for the cross-product term represents the interaction contrast (IC), that is, the difference in the differences of the mean number of cigarettes smoked per day when the risky environment (increased number of traumatic events, decreased neighborhood cohesion and increased neighborhood physical disorder) is present versus absent for those with and without increased GRSs. χ2 tests were used to assess the statistical significance of the IC
