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The British Journal of Psychiatry (1975) 126: 185-190. doi: 10.1192/bjp.126.2.185
© 1975 The Royal College of Psychiatrists
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Independent Transmission of IQ and Schizophrenia

MARSHALL B. JONES M.A., Ph.D.1 and DAVID R. OFFORD M.D., C.R.C.P.(C.)2

1 Professor of Behavioral Science, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033
2 Professor of Psychiatry, University of Ottawa, Director of Clinical Services, Children's Division, Department of Psychiatry, Royal Ottawa Hospital, 1145 Carling Avenue, Ottawa K1Z 7K4, Ontario, Canada

Low IQ is a poor prognostic sign in schizophrenia, and pre-schizophrenics tend to have lower childhood IQs than their siblings; but the explanation for these relations is uncertain. The reduced IQ may be a prodromal or concurrent effect of the schizophrenia; or the schizophrenia may be partially due to the low IQ. A decision hinges largely on the correlations for IQ between schizophrenics and their siblings. If the childhood IQs of schizophrenics are lowered relative to their sibs as a prodromal effect, then the proband-sibling correlation should be substantially reduced and certainly lower than the within-sibling correlation. On the other hand, if IQ and schizophrenia are independently transmitted, the proband-sibling correlation should be essentially the same as in families with no schizophrenics. In this study the childhood IQs of 85 schizophrenics and their 208 siblings were collected from the subjects' school records. The correlation between the probands and their oldest sib was .43, between the probands and their average sib .44, while the intraclass correlation among the sibs was .33. These results concord well with those generally reported for sibling-sibling correlations. The evidence, therefore, supports independent transmission and the view that low IQ functions as an unfavourable, largely genetic modifier in schizophrenia.

Submitted on April 1, 1974




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