To provide a uniform approach to analysis consistent with the variable definitions of the Caspi et al study,10 the variables of each of the studies were coded as follows: major depression as a dichotomous outcome when based on diagnostic interviews, or standard cut points or above the 85th percentile of the scales’ score distribution when based on dimensional measures; stressful life events (or their equivalent) were coded on an ordinal scale (0, 1, 2, ≥3); and the 5-HTTLPR genotype was coded as 0, 1, or 2 for the number of S alleles. Sex, age, and twin status were also coded. Our coding differs from that of some of the original studies that used a different coding strategy such as dichotomized high and low stress (eg, Taylor et al26) or multiple types of environmental stressors (eg, Eley et al18) rather than the ordinal scale of life events that was included in most studies.42,43
