Genetic epidemiology is a relatively new discipline that seeks to elucidate the role of genetic factors and their interaction in the occurrence of disease in populations (Khoury et al., 1993). It is becoming more apparent that most diseases are not purely genetic or purely environmental in origin but depend on a complex interaction of the two. Even with ‘infectious’ or ‘environmental’ aetiology, differential genetic susceptibility may be involved in determining the ultimate clinical manifestation.

Historical observations that schizophrenia runs in families led to a number of family, twin and adoption studies, which provide strong evidence for a genetic component to the disorder. Cardno and Murray review these ‘classical’ genetic epidemiological studies in Chapter 10, with particular emphasis on the twin study approach. Although the size of the genetic contribution is still debated, even conservative estimates suggest heritabilities of greater than 60%. A polygenic model is most likely, but the question remains about what these individual genes might actually transmit. Cardno and Murray suggest that some families transmit a liability to traits for minor deviations that are relatively innocent in themselves but, in combination with other genetic or environmental risk factors, may propel an individual past a threshold for the expression of symptoms.

Zammit, Lewis and Owen take these issues further in Chapter 11 by discussing the methods that can be used to search for ‘genes for schizophrenia’ in the future.