In addition to finding specific genes, molecular genetics can provide information about the heritability of psychiatric disease, an approach that has led to some important insights about the genetic architecture of psychiatric illness. The degree of SNP sharing among disease cases estimates the common, inherited portion of a trait (20). Such SNP heritability estimates can be used to test hypotheses about the extent to which heritability arises from many loci of small effect (“polygenicity”). Using this approach, a large proportion of the genetic contribution to psychiatric disease is found to consist of common variants at a large number of loci, although each variant has only a small effect on disease risk, consistent with findings from other common, complex diseases (21). Thus, a major component of risk of psychiatric illness is polygenic. At the same time, the SNP heritability does not explain all of the estimated additive heritability, suggesting that other, as-of-yet–unmeasured factors such as rare variants, also contribute.
