In order to test the hypothesis that the G×E is specific to persistent depression, we compared results for persistent depression with results for single-episode depression (i.e., depression diagnosed on only one of the four measurement occasions in each cohort) (Fig. 1). In Dunedin, 208 individuals (24.6%) fulfilled criteria for single-episode depression. In E-Risk, 171 (18.4% of the sample) had single-episode depression. Single-episode depression was unrelated to 5-HTTLPR genotype (Table 1). The relationship between single-episode depression and childhood maltreatment was weaker and inconsistent across cohorts (Table 2). None of the G×E tests with single-episode depression as the outcome showed significant results (smallest P=0.099), and there was no significant interaction effect even when the samples were combined with the Fisher’s trend method (Table 3). Examination of estimates and confidence intervals reveals that the differences between results for single-episode and persistent depression are substantial, with estimates for single episode depression falling outside the 95% confidence intervals of estimates for persistent depression in three out of the six comparisons (Table 3).
