Abstract
BACKGROUND: Schizophrenia and psychosis have high twin heritability (approximately 80%), but these general estimates may hide aetiological variation. This can be investigated by using syndromes based on key psychotic symptom combinations. AIMS: To investigate concordance, and heritability where calculable, of psychotic syndromes in multiple schizophrenia and psychosis twin samples. METHOD: We investigated concordance for positive, negative and disorganised psychotic syndromes, based on lifetime symptom ratings, in three classical schizophrenia twin samples (Fischer, Kringlen and Slater) and two psychosis samples (Maudsley register and non-register), the first four being systematically ascertained (total 317 monozygotic and 145 dizygotic probandwise pairs). We assessed concordance differences with logistic regression in generalised linear mixed models, and heritability from twin-modelling in the Maudsley register sample. RESULTS: The positive syndrome, comprising delusions plus hallucinations, had 37.7-41.4% monozygotic and 6.0-6.3% dizygotic concordance, and heritability of 0.81 or 81% (95% CI 0.58-0.88), with similar results for negative and disorganised syndromes. In the systematically ascertained samples, delusions and hallucinations occurring without disorganised symptoms had nominally lower monozygotic twin concordance than when disorganised symptoms were also present (in the three schizophrenia samples: 89 pairs, odds ratio 3.47 (95% CI 1.04-1.54), p = 0.043; and the Maudsley register psychosis sample: 70 pairs, odds ratio 7.68 (95% CI 1.49-39.70), p = 0.016). CONCLUSIONS: In schizophrenia and psychosis, the syndrome of delusions plus hallucinations has high twin heritability overall. Positive symptoms without disorganised symptoms may indicate relatively high environmental influences, and positive symptoms with disorganised symptoms, relatively high familial and probably genetic influences, but further confirmation is needed.