Our results show direct, empirical, quantified molecular evidence for an important genetic contribution to the five major psychiatric disorders. The hSNP2 estimates for each disorder—schizophrenia, 0.23 (0.01 s.e.), bipolar disorder, 0.25 (0.01 s.e.), major depressive disorder, 0.21 (0.02), ASD, 0.17 (0.02 s.e.) and ADHD, 0.28 (0.02 s.e.)—are considerably less than the heritabilities estimated from family studies (Table 1). Yet, they show that common SNPs make an important contribution to the overall variance, implying that additional individual, common SNP associations can be discovered as sample size increases50. hSNP2 values are a lower bound for narrow-sense heritability because they exclude contributions from some causal variants (mostly rare variants) not associated with common SNPs. Although SNP-based heritability estimates are similar for major depressive disorder and other disorders, much larger sample sizes will be needed, as high risk for a disorder implies lower power for equal sample size51. The hSNP2 values are all lower than those reported for height (0.45, 0.03 s.e.)39, but the estimates are in the same ballpark as those reported for other complex traits and diseases using the same quality control
