Age of Onset of Schizophrenia in Siblings: a Test of the Contagion Hypothesis

The possibility that schizophrenia is horizontally transmitted has been assessed in an analysis of age of onset in 264 recorded pairs of siblings with the disease. Age of onset was found to be correlated between siblings, and there was a tendency for the disease to occur at an earlier age in the younger sibling. Three explanations for this finding are considered: horizontal transmission, early detection, and ascertainment bias. An analysis by date of birth differences between siblings gives results consistent with horizontal transmission, but analysis by order of onset of illness (which shows that the age shift is not seen in elder-sibling-ill-first pairs) indicates that ascertainment bias (which arises from a tendency to include an excess of early onsets in younger siblings) is a more cogent explanation of the age shift. Although horizontal transmission is not altogether eliminated, the data suggest that age of onset is determined by genetic or prenatal factors rather than environmental precipitants in postnatal life. The retrovirus/transposon hypothesis (Crow, 1984) can accommodate the findings more readily than the gene-virus interaction hypothesis (Crow, 1983).