To understand the extent of improper usage of covariates in G×E studies, I selected all (47) novel G×E studies that were identified in the Duncan & Keller (2011) review of the first ten years of candidate G×E studies in psychiatry. Novel studies (first reports of a given G×E finding) were selected because replication attempts were likely to employ the same model used in the original report, and therefore would provide redundant information about typical practices for controlling covariates. Studies were coded according to the following criteria: (a) whether they reported significant G×E findings or not; (b) whether the investigators properly controlled for covariates by including all relevant covariate-by-gene and covariate-by-environment interactions; and (c) whether the sample was ethnically heterogeneous or not. Of the 47 studies, 45 (96%) reported significant G×E results (Table 1). This high rate, when compared to the lower rate (27%) of positive results among replication attempts (not shown), is probably symptomatic of publication bias (1). As shown in Table 1, of the 41 studies that attempted to statistically control for potential confounders by including them as covariates
