Searching for the split in schizophrenia: a twin study perspective.

The characteristics of two subtypes of schizophrenia (here called "H" and "P" types) that were derived in a previous study were further examined using discriminant analysis. The scores on the resultant discriminant function were used to assign Gottesman and Shields' (1972) sample of schizophrenic twins to subtypes. Although there was a tendency for cotwins to be assigned to the same subtype as the proband, the degree of homotypia in monozygotic twins, at 73.3%, was not perfect. A diagnosis of schizophrenia (of either H or P type) was significantly more common in the cotwins of H than P type probands. The findings suggest that the two subtypes are not genetically distinct conditions, but are more likely to represent varieties of disorder that occupy different positions on the same multifactorial continuum of liability.