the same risk as the children of the affected twin. The risk in the offspring of schizophrenic identical twins is 16.8 percent; it is 17.4 percent in the unaffected twins' offspring (Gottesman and Bertelsen, 1989). This evidence that inheritance can “pass through” apparently asymptomatic, unaffected individuals suggests a strong genetic component but again clearly indicates that nongenetic factors are also involved.
Adoption Studies. Two strategies have been used to estimate genetic effects in adoptive families. One approach is to evaluate offspring of schizophrenic parents who have been adopted away from their biological parents into another environment and measure the rates at which such children develop schizophrenia. The second approach is to study the rearing, adoptive families of adopted children who have developed schizophrenia as adults. Heston (1966) studied adopted-away offspring of schizophrenic mothers and found a significant rate of schizophrenia in these children (10.4 percent prevalence, which became 16.6 when necessary age corrections were made), compared with children from unaffected mothers (no children with schizophrenia). A study by Rosenthal (1974) showed a similar result (13.9 percent from biologically affected families and 3.4 percent of controls), but the data did not achieve statistical significance. In a much larger Finnish study (Tienari, 1991a,b,c), 10.3 percent of schizophrenic women's children also became schizophrenic, even when raised by adoptive families, confirming Heston's results. Kety and co-workers, on the other hand, began their study with adoptees who grew up to be schizophrenic (Kety, Rosenthal, Wender, Schulsinger, and Jacobsen, 1978). The adoptees' adoptive and biological families were located and evaluated. Both schizophrenia and the schizophrenic spectrum disorders were significantly higher in the biological relatives than in the adoptive families. These results have been subject to extensive reanalysis, including careful diagnostic reevaluations (Kendler and Gruenberg, 1984). Taken as a body of work, these studies collectively indicate that the offspring of schizophrenic individuals have a greatly increased risk of developing schizophrenia and that schizophrenics do not transmit this vulnerability to their nonbiological families. This evidence points to a strong role for genetics as a risk factor. The genetic data also make clear that nongenetic factors, such as environmental influences and acquired brain damage, must have a role in accounting for the incidence of the disease (Gottesman and Bertelsen, 1989).
Linkage Studies. Linkage studies in schizophrenia using conventional markers and assuming a single major locus have been inconclusive.