The authors synthesize findings of previous studies implicating migration as a risk factor for the development of schizophrenia and provide a quantitative index of the associated effect size. MEDLINE was searched for population-based incidence studies concerning migrants in English-language publications appearing between the years 1977 and 2003. Article bibliographies and an Australian database were cross-referenced. Studies were included if incidence reports provided numerators and denominators and if age correction was performed or could be performed by the authors. Relative risks for migrant groups were extracted or calculated for each study. Significant heterogeneity across studies indicated the need for a mixed-effects meta-analytic model. The mean weighted relative risk for developing schizophrenia among first-generation migrants (40 effect sizes) was 2.7 (95% confidence interval [CI]=2.3-3.2). A separate analysis performed for second-generation migrants (seven effect sizes) yielded a relative risk of 4.5 (95% CI=1.5-13.1). An analysis performed for studies concerning both first- and second-generation migrants and studies that did not distinguish between generations (50 effect sizes) yielded a relative risk of 2.9 (95% CI=2.5-3.4). Subgroup comparisons yielded significantly greater effect sizes for migrants from developing versus developed countries (relative risk=3.3, 95% CI=2.8-3.9) and for migrants from areas where the majority of the population is black (relative risk=4.8, 95% CI=3.7-6.2) versus white and neither black nor white. A personal or family history of migration is an important risk factor for schizophrenia. The differential risk pattern across subgroups suggests a role for psychosocial adversity in the etiology of schizophrenia.
