One thing that is certain about the aetiology of schizophrenia is that there is no single cause. This might reflect the fact that the schizophrenia construct itself is heterogeneous, such that specific subtypes might in the future be found to have specific causes. But it is more useful at this stage of our knowledge to conclude that, like other disorders such as ischaemic heart disease and diabetes mellitus, schizophrenia results from the cumulative effects of a number of risk factors. These may be crudely divided into the familial-genetic and the environmental, though there are clearly interactions between the two.