Concordance rates and biometrical model fitting for operational diagnoses in the maudsley twin psychosis series

Background: Twin studies have supported a genetic contribution to the major categories of psychotic illness, but few of these have employed operational diagnostic criteria, and no study using such criteria has been based on a sample that includes the full range of functional psychotic illnesses. Objectives: To investigate concordance rates and perform biometrical model fitting for operationallydefined psychotic diagnoses in a sample of twins with any functional psychosis. Methods: 224 twin probands (106 MZ, 118 DZ) with a same-sex co-twin and a lifetime history of psychotic symptomatology were ascertained from the service-based Maudsley Twin Register, in London. RDC psychotic diagnoses were made on a lifetime-ever basis. Main-lifetime diagnoses of DSMIIIR and ICD10 schizophrenia were also made. Probandwise concordance rates and correlations in liability were calculated, and biometrical model fitting applied. Results: A genetic contribution to variance in liability was confirmed for the major diagnostic categories except RDC depressive psychosis and unspecified functional psychosis, where familial transmission was confirmed, but the relative contribution of genetic and common environmental factors was unclear. Heritability estimates were: 82% for RDC schizophrenia; 85% for RDC schizoaffective disorders; 83% for all RDC affective psychoses; 84% for RDC mania; 89% for all RDC psychotic diagnoses; 84% for DSMIIIR schizophrenia; and 83% for ICD10 schizophrenia. Conclusions: Heritability estimates for schizophrenia, schizoaffective disorders, mania, and all psychoses combined were substantial and similar. Population morbid risk estimates were inferred rather than directly measured, but the results were very similar to those from studies where morbid risks were directly estimated.