Finally, although no study included all relevant covariate-by-environment or covariate-by-gene terms to control for the effects of the covariates on G×E interactions, it should be noted that several studies conducted follow-up analyses that went at least partway toward eliminating certain covariates as alternative explanations for the G×E findings. Dick et al. (26) tested gender-by-gene and age-by-gene interactions in separate models that did not include the G×E term and found they were not significant. This procedure does make it less likely that the two covariates investigated are responsible for the reported G×E interaction. However, it did not include the covariate-by-gene or covariate-by-environment interactions in the primary model, and as noted above, even non-significant covariate interaction terms can substantively change conclusions about the interaction of interest. Caspi et al (27) stratified their sample by MAOA genotype and noted that the interaction held in each subsample. This is a highly conservative control for MAOA and essentially eliminates it as a potential confounder of their G×E finding, but the investigators did not control for their other covariate (gender) in the same way and did
