Evidence from nearly a century of epidemiological research indicates that schizophrenia occurs in all populations with a prevalence in the range of 1.4 to 4.6 per 1000 and incidence rates in the range of 0.16-0.42 per 1000 population. Multi-centre studies conducted by the World Health Organization have highlighted important differences between 'Western' and 'Third World' populations as regards the course and outcome of the disorder, with a significantly better prognosis in the developing countries. The factors underlying the better outcome of schizophrenia in developing countries remain essentially unknown but are likely to involve interactions between genetic variation and specific aspects of the environment. These features place schizophrenia, along with diabetes, cancer and hypertension, into the group of genetically complex diseases which are characterised by polygenic transmission, locus heterogeneity and environmental contribution to causation. The emerging pattern of risk factors and antecedents of schizophrenia suggests multiple, mainly quantitative deviations from the average developmental trajectory, primarily in the areas of early neurodevelopment, cognitive ability and social behaviour. These deviations are compatible with the notion of non-specific background factors facilitating the operation of genetically determined causal pathways. Research likely to result in new insights should focus on the population distribution and behavioural effects of potential risk factors and markers suggested by biological and genetic research.