Studies have clearly established that the concordance rate for schizophrenia is much higher in MZ than DZ twins. Beyond this, it is difficult to generalize the findings to singletons. Analyzing the reports of MZ twins discordant for schizophrenia, Stabenau and Pollin (1967) view the differences between the index case and his nonschizophrenic cotwin, as stemming from an early biological defect or immaturity, which, in interaction with parental handling and other environmental response, may lead to socio-psychological incompetency and, in turn, to gross schizophrenic symptomatology.

In a previous study (Pollack et al, 1966), comparing schizophrenic patients and their nonschizophrenic sibs, we found that patients showed significant psychological (social and personality) deviations from the preschool age through adolescence. We did not obtain significant differences on the biological and very early developmental variables that clearly distinguished the discordant twins. These include birth weight, paranatal complications or motor development, such as age of walking. In contrast, Lane and Albee (1966) found schizophrenic index cases to be lighter at birth than their sibs, confirming the finding for twins. The clinical significance of this difference is debatable, since the index cases did not differ from national norms.