We first examined the association between the number of stressful life events compared with none as the explanatory variable with depression as the response variable. For each number of stressful life events, we calculated an odds ratio (OR) and its 95% confidence interval (CI). Second, we investigated the associations between stressful life events alone (entered as an ordinal variable) and the number of S alleles alone on depression as the response variable. Third, we examined the joint effect of number of stressful life events and number of S alleles on the risk of depression to test whether there is an interaction in their influence on the risk of depression, as indicated by a significant OR for their joint effect on the response variable of depression. Fourth, to determine whether gene-environment correlation exists,we evaluated the association between the proportion of S alleles (dependent variable) and number of stressful life events (independent variable) among those without depression by logistic regression. As another test of gene-environment interaction, we performed the same regression as above in those with depression instead of those without depression and then calculated the difference (δ) in β coefficients and the standard error of δ.
