Nature and nurture interplay: schizophrenia.

There is compelling evidence from family, twin and adoption studies of a substantial genetic contribution to schizophrenia. The mode of transmission is complicated and very rarely if ever involves a single gene. Rather schizophrenia results from multiple genes of small effect and their interplay with the environment. Perhaps because the overall size of the genetic effect is large, accounting for about 80 % of variance, definite environmental factors have been difficult to pin down. It has even been suggested that "the environment" consists entirely of epigenetic or stochastic phenomena that can never be detected by a standard epidemiological methods. Nevertheless, a variety of social stressors, including high expressed emotion in relatives and life events affect the course of illness and certain physical factors such as obstetric complications and cannabis smoking have been implicated in contributing to liability to the disorder. The recent discovery of several positional candidate genes that have been replicated as being associated with liability to schizophrenia holds considerable promise not just for a better understanding of the neurobiology but also for improved knowledge about risk prediction and gene-environment interplay.