An Icelandic Family Study of Schizophrenia

Abstract

Data on the families of schizophrenic index cases in Iceland show the risk of schizophrenia to be raised about four-fold in first degree relatives over that of the population at large. The risk of other forms of functional psychosis in these relatives is also increased by a factor of four. The rates of psychosis in more distant relatives appear to be somewhat higher than previous investigators have reported. A comparison of the new data with the older studies leads to the conclusion that other authors have probably counted all functional psychosis in close relatives as schizophrenia, thus arriving at a risk of approximately 15 per cent in first degree relatives. In the present study a rate similar to this figure is obtained when all functional psychoses are combined. The new Icelandic data fit best a genetic hypothesis based on one major gene, partially dominant. Recent twin studies claiming to have established that previously reported concordance rates for schizophrenia are too high, are criticized, and it is pointed out that these findings are based on an error in the calculation procedure.