A Study of Pairs of Sibs both Hospitalized for Mental Disorder

¹ Visiting Research Worker, M.R.C. Psychiatric Genetics Research Unit, Institute of Psychiatry, Maudslay Hospital, London, S.E.5., Lecturer and Senior Psychiatrist, Department of Neurology and Psychiatry, National Taiwan University Hospital, Taipei, Taiwan, Republic of China (Formosa)

The present study is concerned with the problem of what is inherited in mental disorder. This is the first study of its kind in which pairs of sibs both hospitalized for any mental disorder are analysed according to consistent independent diagnosis and according to separate clinical features, together with full presentation of case histories (Tsuang, 1965).

The medical records have been studied of 71 pairs of sibs hospitalized in Claybury Hospital, Essex, and notified over a 15 year period to the Genetics Unit, Institute of Psychiatry, Maudsley Hospital, London.

On the whole the findings of the present study are consistent with those of previous work, which has been reviewed.

Statistical analysis reveals that:

(1) pairs of sibs are significantly alike in psychiatric diagnosis; less resemblance is found in respect of single symptoms; reliable conclusions about course and outcome cannot be made without follow-up.

(2) there is an excess of same-sexed pairs in the material, but they are no more alike diagnostically than opposite-sexed pairs, and female pairs are no more alike than males; when allowance is made for the different incidence of schizophrenia and affective disorders in males and females, there is still a significant tendency for pairs of sibs to be alike in having ever been given these diagnoses.

(3) the within-pair correlation in age of first admission is around 0.8, and this may in part be accounted for by the fact that pairs of sibs who were in hospital at about the same time were relatively more likely to be notified.

Despite differences in phenotypic manifestation or in classification, it was possible, on clinical analysis, to regard between 68 per cent. and 89 per cent. of sib pairs as suffering from illnesses of a similar genetical kind.

It is concluded that of all the clinical formulations descriptive of mental illness it is the diagnosis which comes closest to the genetical predisposition. Although influenced by environmental factors, the psychiatric diagnosis (and to a lesser extent some other features of the illness) is based on hereditarily determined individual variation. The author considers that the findings of the investigation, though open to error from many sources, tend to support the view that schizophrenia and affective disorders are related to polygenic rather than monogenic genetical causes; but that these polygenic systems are likely to have their main effects on two independent dimensions.

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