A genetic component to the aetiology of schizophrenia was established early in this century (Jablensky, 1999) but, apart from this, the aetiology still remains something of a mystery. In recent years, epidemiologists have tended to study environmental factors associated with disease (Ch. 5) whereas geneticists have focused on the application of model-fitting approaches to try to quantify the genetic component (Ch. 10) or on developing specialized methods to map and identify susceptibility genes. However, it is becoming increasingly clear that the identification of specific genes that increase risk factors will be greatly aided by the appropriate application of epidemiological principles. Moreover, geneticists and epidemiologists will need to work even more closely together once such genes are identified in order to establish the impact of specific genetic risk factors on populations and to study the interactions of these factors with each other and with environmental factors.

Schizophrenia is a complex, multifactorial condition that results from the influence of both genetic and environmental factors. Our understanding of other complex disorders has shown us that it is more productive to consider how these factors might be operating together to bring about disease rather than to classify them as distinct aetiological processes, since identified risk factors appear to be neither sufficient, nor always necessary, to cause disease. Additionally, the division between genetic and environmental causation is not always clear.