Editors' introduction Epidemiological studies related to the field of migration have demonstrated that rates of psychotic disorders among some migrant groups are higher than expected. In Chapter 2, Kirkbride and Jones illustrated this by proposing hypotheses to explain such variation and assessing each hypothesis. Liu and Cheng focus on some of the methodological issues related to research design, time of assessment, cross-sectional nature of these studies and other factors. Issues related to refugees and asylum seekers are somewhat different because of the trauma they may suffer and the application of the stress-diathesis model here. As expected, post-traumatic stress disorders will show a higher prevalence among refugees and asylum seekers. Methodological, especially sampling, problems in these groups raise issues about the implications of such studies. Liu and Cheng also explore the factors related to returning migrants and internal migration. They conclude that both social and genetic factors need to be understood in explaining the variation in rates, and early life experiences may determine the outcome of mental illness in the new settings. Using a number of factors in identifying aetiology will also enable clinicians to develop appropriate management plans. Introduction One of the major aspects of social changes following rapid globalisation from the second half of the last century is migration. According to the International Organization for Migration, there are now about 192 million people living outside their place of birth, which is about 3% of the world's population (http://www.iom.int/jahia/Jahia/about-migration).
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