The heritability of MDD is 31–42%,10 although certain subsets of MDD may be more heritable (for example, recurrent, early-onset MDD or clinically ascertained MDD).11,12 The modest heritability of MDD could reasonably be expected to complicate attempts to identify genetic loci that confer risk or protection. However, heritability is not necessarily a key determinant for the identification of strong and replicable genetic associations.13 For example, there have been notable successes in genome-wide searches14 for susceptibility loci for breast cancer (heritability ~25%), lung cancer (26%), Type 2 diabetes mellitus (26%), Parkinson’s disease (34%), multiple sclerosis (41%), systemic lupus erythematosus (44%) and age-related macular degeneration (46%).15–20
