Evidence for polygenic inheritance in schizophrenia?

The frequency of psychotics among sibs of index cases with a functional psychosis was found by Odegaard (1972) to be positively associated with the number of psychotics in the parents' generation. Odegaard considered this correlation to be indicative of polygenic inheritance. However, an attempt at replicating this fascinating finding in a sample of schizophrenic families was not successful. Although this discrepancy in results might have been due to chance, the importance of the problem of the mode of inheritance induced a search for biases that might have obscured the comparison of the two samples. Unfortunately, the discrepancy remained unexplained, but certain structural and diagnostic differences were brought up and discussed. The very existence of such incongruities, together with the contrasting main results, seems to caution against definite genetic conclusions until further evidence is available.