Our sample comprised the 1868 participants with bipolar disorder and 2938 controls investigated in the WTCCC study. A detailed description of the sample has been provided elsewhere.3 All individuals were from the UK and over the age of 16 years. Clinical assessment included semi-structured interview and review of case notes. Ratings of symptom occurrence and course of illness were made including the operational criteria (OPCRIT) item checklist.8,9 Diagnoses were based on all available data. The primary diagnostic system used for classifying participants was the Research Diagnostic Criteria (RDC)10 because it provides more delineation between individuals on the basis of the pattern of mood and psychotic symptomatology than do DSM–IV11 or ICD–10.12 Participants with bipolar disorder had experienced at least one episode of clinically significant elevated mood according to RDC: bipolar I disorder (n = 1316), schizoaffective disorder, bipolar type (n = 279), bipolar II disorder (n = 171) and manic disorder (n = 102). The diagnoses according to DSM–IV for the same set of participants were: bipolar I disorder (n = 1594), schizoaffective disorder, bipolar type (n = 98), bipolar
