Overwhelming evidence purports that genetic mechanisms influence the propensity to develop schizophrenia. .
Family pedigree studies demonstrate that the risk of developing schizophrenia increases the more closely one is related to someone who already has the illness. For instance, first-degree relatives have a 10% risk whereas for first cousins the risk is reduced to 3%. (Kendler et al 1994).
However, this study can be criticised because it fails to take into account of the shared environment: after all, closely related people tend to live together. However, were genetic mechanisms not involved, it is hard to see why cousins, aunties and uncles should have a 3-fold greater risk, when they usually live away from the person with schizophrenia. Therefore, in general, pedigree studies have provided supportive, but not conclusive, evidence for the role of genetic factors.
A second approach to understanding the contribution of genetic mechanisms is by looking at adoption studies. If there were no genetic factors involved, then we would expect that the risk of developing schizophrenia would be much less in those children adopted away than the children continuing to live with their affected parent(s). In fact, Rosenthal (1971) have shown that there is the same risk to children who have been adopted away from the affected parent, which provides strong evidence for the operation of a genetic mechanism.
Furthermore, there have also been studies of the children of healthy people who were adopted by parents who themselves had schizophrenia (e.g. Wender et al, 1974). If genetic factors were important then we would predict that adoption by ill parents should not increase the risk. Wender's results support this notion, indicating that growing up in proximity to a person with schizophrenia is not in itself a significant risk factor for the later development of the illness.
Twin studies suggest that having an MZ twin with schizophrenia is one of the highest risk factors of all: 48% of all co-twins are also affected.