The evidence for heritability of the major psychiatric disorders comes from the collection of family, twin, and adoption studies, extending throughout the 20th century. This evidence is substantial for schizophrenia, bipolar disorder, major depression, alcoholism, antisocial personality disorder, and autism spectrum disorders, among other conditions. These data are summarized in many standard psychiatric texts (eg, The Medical Basis of Psychiatry33). Heritability is classically calculated as the comparison of monozygotic to dizygotic twin concordance rates, although it can be estimated from comparing other family relationships as well. Also important is the concept of relative risk (quantified by lambda), that is, the increased vulnerability of some individuals with a specific risk factor for a disorder compared with the general population. The lambda for the relationship between risk in first-degree relatives of a proband and individuals in the general population34 for schizophrenia is about 10 and for bipolar I disorder, 8–10.
