Previous reports have indicated that weight at birth was lower for schizophrenic index cases than for their non-schizophrenic monozygotic co-twins; there have been conflicting reports concerning lower birth weights for schizophrenic singletons compared with their siblings.
In this study hospitalized schizophrenics (n = 34) and personality disorders (n = 52) were compared with their own siblings on measures of birth weight and length obtained from medical records. The results indicated no significant differences between patients and their sibs on either measure for either diagnostic group, and no differences between diagnostic groups. However, schizophrenics weighed nearly six ounces less than their same-sex siblings and this difference approached statistical significance.
Differences between these findings and previous studies are discussed in terms of sample differences, and the questionable comparability of twin and sibling studies of birth weight.