On the whole, these results for MDD are in sharp contrast to the now substantial experience with GWAS for other complex human traits. GWAS has been a widely applied (> 860 studies) and remarkably successful technology in the identification of > 2200 strong associations for a wide range of biomedical diseases and traits.14 The vast majority of GWAS with sample sizes > 18 000 found at least one genome-wide significant finding (178/189 studies, 94.2%),14 and yet we found no such associations for MDD. What implications do these null results have for research into the genetics of MDD? Why might the results have turned out this way? We frame our discussion around a series of implications and hypotheses for future research.
