background There is evidence of an increased incidence of schizophrenia in immigrants.
aim To provide a quantitative index of the effect size associated with immigration.
method   We performed a meta-analysis of English-language incidence studies that were published between 1977 and 2003.
results   The mean, weighted relative risk (rr) was 2.7 (95%- confidence interval (ci) 2.3- 3.2) for first-generation immigrants and 4.5 (95%-ci 1.5-13.1) for second-generation immigrants. An analysis of studies relating to both first and second-generation immigrants and studies that did not distinguish between generations yielded a mean rr of 2.9 (95%-ci 2.5-3.4). On comparing the subgroups we found that the effect sizes were significantly greater for immigrants coming from developing countries rr = 3.3; 95%-ci 2.8-3.9) than for those from developed countries, and for immigrants coming from areas where most of the population is black (rr = 4.8; 95%-ci 3.7-6.2) than for those where most people are white or neither black nor white.
conclusion   A personal or family history of immigration is an important risk factor for schizophrenia. The differential risk pattern across subgroups suggest that factors in a person's social surroundings are important for the aetiology of schizophrenia.