Theoretical Commentary on Gottesman and Shields' Review

Gottesman and Shields (1976) state: "When the common parent is unaffected [by schizophrenia], we would expect the risk [to half siblings] to approach the population base rate" (p. 368). I have carried out calculations, based on James' (1971) version of the single major locus model, of the expected incidence of schizophrenia in half siblings when the shared parent is affected or unaffected. A population base rate of .87 percent and an incidence in offspring and in siblings of 10.4 percent were assumed. The results are shown in table 1.