The pathogenic mechanisms of psychiatric disorders are largely unknown, so diagnostic boundaries are difficult to define. Genetic risk factors are important in the causation of all major psychiatric disorders,2 and genetic strategies are widely used to assess potential overlaps. The imminent revision of psychiatric classifications in the Diagnostic and Statistical Manual of Mental Disorders (DSM) and the International Classification of Diseases (ICD) has reinvigorated debate about the validity of diagnostic boundaries. With increasing availability of large genome-wide genotype data for several psychiatric disorders, shared cause can now be examined at a molecular level.
