Mental disorders are among the most heritable complex human disorders. For schizophrenia, bipolar disorder (BIP), major depression (MD) and attention-deficit hyperactivity disorder (ADHD), twin studies have estimated broad sense heritabilities of 40–80%.1–4 Genome-wide association studies (GWAS) have since shown that approximately one third is driven by thousands of common genetic variants, each with individually small effect.5 As ever-expanding GWAS sample sizes bring genetic prediction and stratification tools closer to the clinic,6 a better understanding of the complex genetic architecture of mental disorders is needed to address questions regarding disease classification and the potential for precision medicine in psychiatry.
