Heritability estimates for psychotic disorders - The Maudsley Twin psychosis series

Background: Previous twin studies have supported a genetic contribution to the major categories of psychotic disorders, but few of these have employed operational diagnostic criteria, and no such study has been based on a sam ple that included the full range of functional psychotic disorders. Methods: A total of 224 twin probands (106 monozygotic, 118 dizygotic) with a same-sex co-twin and a lifetime history of psychosis was ascertained fro m the service-based Maudsley Twin Register in London, England. Research Dia gnostic Criteria psychotic diagnoses were made on a lifetime-ever basis. Ma in-lifetime diagnoses of DSM-III-R and International Statistical Classifica tion of Diseases, 10th Revision schizophrenia were also made. Pro-bandwise concordance rates and correlations in liability were calculated, and biomet rical model fitting applied. Results: A substantial genetic contribution to variance in liability was co nfirmed for the major diagnostic categories except Research Diagnostic Crit eria depressive psychosis and unspecified functional psychosis, where famil ial transmission was confirmed, but the relative contribution of genetic an d common environmental factors was unclear. Heritability estimates for Rese arch Diagnostic Criteria schizophrenia, schizoaffective disorder, mania, DS M-III-R schizophrenia, and International Statistical Classification of Dise ases, 10th Revision schizophrenia were all between 82% and 85%. None of the estimates differed significantly from any other. Conclusions: Heritability estimates for schizophrenia, schizoaffective diso rder, and mania were substantial and similar. Population morbid risk estima tes were inferred rather than directly measured, but the results were very similar to those from studies where morbid risks were directly estimated.