to account for the lower heritability of MDD (0.37)10 compared with schizophrenia (0.81),88 implying lower effect sizes at each locus (see Wray et al.25 and Yang et al.87 for details). Obtaining a total sample size on the order of 100 000 MDD cases plus controls would require a significant investment for ascertainment, phenotyping, DNA collection and genotyping, but could be accomplished using national registers or via electronic medical records of large health care organizations. Such sample sizes have been achieved in studies of quantitative traits and yielded large numbers of genome-wide significant results.29,30
