We note that classical family, twin and adoption genetic studies have long been used as one of the key methods for validating psychiatric diagnoses.16 Classical approaches require twin and family samples, the availability of which is limited by the substantial difficulties and high costs inherent in their recruitment. Classical genetic studies estimate the contribution of all inherited forms of variation. Here we have used available genome-wide association study data to estimate the relative contribution of the common genetic variation to susceptibility to different phenotypes. This may be a useful approach for delineating phenotypic sets that could be particularly fruitful for study using genome-wide association studies. The major limitation of using genome-wide association study data is that current genome-wide association study genotype data-sets do not capture all genetic variation that may be relevant to illness. Rather, they provide information about a substantial proportion of common genetic variation within the genome. They do not currently provide information about the contribution of rare variants or mechanisms such as structural variation.
