Inbreeding and schizophrenia.

The unique situation that the Norwegian 1891 census included information on consanguineous relation between spouses and that first admissions to psychiatric hospitals by diagnosis were available for the years 1921-40, formed the basis for the present study. There are similarities between the pattern of inbreeding and admission rates in our rural communities, but we were unable to demonstrate any significant correlation. This is interpreted as due to selective avoidance in the choice of a marriage partner, particularly between near relatives. Good neighbourhood knowledge was an important factor in our sparsely populated communities with a marriage pattern distinguished by extreme geographical proximity of residence of spouses. The considerably lower proportion of first cousin matings than expected among parents of psychiatric in-patients 1926-55, illustrates the lower psychiatric morbidity in offspring of consanguineous parentage. Admissions 1921-40 comprised only functional psychoses with a predominance of schizophrenia (greater than 80%), which today is diagnosed in less than 10%. A decline in incidence rate is not likely, whereas changing diagnostic practice is probably a main factor. The near disappearance of catatonia and hebephrenia could be related to the introduction of psychotropic drugs and the accompanying improvement in hospital treatment. The epidemiology of schizophrenia is unchanged with a significant excess in lower social classes. This could in part be due to schizophrenic phenocopies related to the unchanged social differential in infant mortality and morbidity (including a particular season of birth effect), in part to the prevalence of adverse environment postnatally. The considerably greater male marital differential has probably a social cause, whereas male earlier onset of disease and more severe course of the disease are discussed in relation to the new concept of progressive sexual brain differentiation.