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There is now compelling evidence that migrant groups in several countries have an elevated risk of developing schizophrenia and other psychotic disorders. Though the findings of earlier studies were greeted with skepticism, and ascribed by some to have methodological shortcomings and diagnostic biases, the more rigorous recent studies, from a variety of countries, have still found markedly increased incidence rates. While this phenomenon is an important health issue in its own right, understanding the reasons for the increased rates may provide valuable insights into the causes of schizophrenia and other psychotic disorders in general. The challenge for the next phase of studies is to identify the relevant risk factors and how they might interact to increase the risk of psychosis, both in migrant groups and in the general population.
The first important studies of schizophrenia in migrant groups were carried out in the United States in the first half of the twentieth century. Ødegaard's1 landmark study of Norwegian migrants to the United States demonstrated a 2-fold increase in first admission rates for schizophrenia compared with native-born Americans or Norwegians. Subsequently, Malzberg2,3 also noted higher first-admission rates for schizophrenia among the foreign-born residents of New York State, taking account of differing population age structures and urbanization.
In the 2 decades following the end of the Second World War, there was a large influx of migrants to the United Kingdom from Commonwealth countries, particularly the Caribbean (eg, Jamaica, Barbados, and Trinidad) and the Indian subcontinent. The earliest studies of the incidence of schizophrenia in the new migrant populations reported higher than expected rates in migrants of African-Caribbean origin.4,5 There have subsequently been at least 18 studies investigating this issue, all of which have shown that rates are elevated in the African-Caribbean population relative to the white population (between 2 and 18 times). The validity of these findings has been the subject of intense debate in the UK, a debate partly fueled by the methodological shortcomings of early studies (eg, inadequate population denominator data, use of nonstandardized diagnoses, and uncertainty over the completeness of case ascertainment).
However, many of these early methodological problems have since been overcome. The seminal study by Harrison et al.6 in Nottingham was the first study of its kind that was based on sound epidemiological principles, including prospective case finding within a defined catchment area, standardized assessments of mental state, operational diagnostic criteria, and extensive use of collateral history.7 They found that the incidence rate of schizophrenia was raised more than 12-fold in the African-Caribbean community, compared with the general population.
Even more robust are the 4 studies that used the 1991 UK census data to estimate population denominators for each ethnic group. This census was the first to collect comprehensive data on the ethnic composition of the general population. Three of these studies employed a prospective design and ascertained cases by first contact of services. The studies by Bhugra et al.8 and Harrison et al.9 both employed a similar methodology for case ascertainment and assessment as the World Health Organization 10-country study.10 The incidence rates of schizophrenia in these studies were raised in the African-Caribbean population; even continuing uncertainties in the denominator could not account for the findings. King and colleagues11 found raised rates of schizophrenia not only in African-Caribbeans but also in all ethnic minority groups studied, although these findings were based on relatively small numbers of cases. Similarly, van Os and colleagues,12 employing a case register approach, found increased rates of schizophrenia for both African-Caribbeans and black Africans (ie, people of sub-Saharan African origin, a more recent migrant group to the UK).
More recently, evidence of similarly raised incidence rates has come from a number of studies performed in other countries in Europe on other migrant groups. These suggest that the rate of schizophrenia in migrants to these countries is also raised. Two studies of migrants to the Netherlands from the Surinam, Dutch Antilles, and Morocco13,14 and 1 study of immigrants to Sweden from East Africa15 demonstrate a higher incidence of schizophrenia in these groups.
The UK-based AESOP study,16 conducted over a 2-year period in 3 UK sites simultaneously (London, Nottingham, and Bristol), is the largest study to date to investigate this issue. It found that rates of schizophrenia were markedly elevated in both African-Caribbeans (rate ratio = 9) and black Africans (rate ratio = 6), in both sexes and across all age groups. Rates for Asian and other groups showed more modest increases, displaying a 2- to 3-fold increase in rates. It is important to note, moreover, that the more recent studies are primarily of second-generation migrants, that is, of those born in the UK to migrant parents.
Although the majority of such studies have concentrated on schizophrenia, some studies have examined the rates of other psychoses in migrant groups. Overall, the evidence points to the rates for other psychoses being raised. Leff et al.,17 Bebbington et al.,18 and Hunt et al.19 all demonstrated more mania in the African-Caribbean population in the UK. A middle syndrome between this and schizophrenia (schizomania) has also been reported as more common.20 Furthermore, Selten and colleagues14 demonstrated an increase in all psychotic disorders in Caribbean migrants to the Netherlands. Selten and colleagues21 also examined the admission rates for both first-episode bipolar disorder of manic and depressed types in migrant groups in the Netherlands and found only small increases in the admission rates for manic illness in Surinamese, but not Turkish, migrants. However, they found more substantial increases in admission rates for both Surinamese and Turkish migrants for bipolar disorder depressed type, particularly for men. Finally, the AESOP study22 demonstrated a 7-fold increase in mania among African-Caribbeans and black Africans compared with whites, and a 2- to 3-fold increase in mania among other migrant groups, findings of the same order of magnitude of those found for schizophrenia in these groups in the same study. Rates for depressive psychosis in the AESOP study were also elevated in all migrant groups, albeit to a more modest degree.16
In their recent meta-analysis of population-based incidence studies of schizophrenia in migrant populations, Cantor-Graae and Selten23 found a mean weighted relative risk (RR) for developing schizophrenia among migrant groups of 2.9 compared with indigenous populations. This effect was more marked in migrants from either developing countries (RR = 3.3) or from countries where the majority population is black (RR = 4.8).
In the United Kingdom most research has focused on the African-Caribbean population. In a separate analysis using data collated for their meta-analysis, Selten (personal communication) found a mean weighted relative risk of 5.0 for African-Caribbeans. Rates appear to be similarly raised in the black African population in the UK. However, the incidence of schizophrenia in other UK migrant groups, particularly Asians, appears to be only modestly raised. Thus, Bhugra and colleagues8 found no overall increase in the incidence of schizophrenia in the Asian population in London, but they did note elevated rates in those over 30 years old. King and coworkers11 found elevated rates in the Asian population in their North London study, and more recently AESOP16 found about a 2-fold increase in the rate of schizophrenia and other psychoses in all Asian groups. What can be stated with confidence is that the rates for schizophrenia and mania in Asians are not raised to the same extent as in African-Caribbeans and black Africans. Understanding differences between groups who migrated to the UK during the same era, and who appear to experience similar levels of racial discrimination and social disadvantage, may reveal important factors that play a part in the raised rates of psychosis in certain ethnic minority groups.
The obvious question from the above findings is whether these raised rates exist in the countries of origin of migrant groups. The evidence that exists to date suggests that the answer is no. Three major incidence studies have been conducted in the Caribbean in the 1990s. They cover Jamaica,24 Trinidad,25 and Barbados,26 3 islands from where the majority of UK migrants came. All employed a prospective design to ascertain all cases making first contact with services. The incidence rates of schizophrenia in Jamaica, Trinidad, and Barbados were found to be more similar to the rate for the white population in the United Kingdom and were significantly lower than the comparable rate for the UK African-Caribbean population. Selten et al.27 further found that incidence rates in the Surinam were significantly lower than among Surinamese migrants to the Netherlands.
The question of whether the raised rates can be explained as methodological artifact has been mentioned above. The sheer weight of evidence is now such that the finding of higher rates of schizophrenia and other psychoses in migrant groups has to be accepted as valid. The question consequently arises: Why?
Ødegaard1 had suggested that a shared predisposition to migration and to schizophrenia might account for the excess of schizophrenia noted in Norwegian migrants in the United States. However, the finding of increased rates of schizophrenia in Surinamese migrants in the Netherlands, combined with the fact that about half the inhabitants of Surinam migrated to the Netherlands, makes selective migration an unlikely explanation.28 Further, the idea that the process of migration (either selection or stress) could, in isolation, explain the raised rates is undermined by the absence of a clear temporal association with migration and onset5 and by the fact that rates are also elevated in the second generation.
Two studies have shown that there is a much higher risk of schizophrenia in siblings than in the parents of African-Caribbean people with schizophrenia, the risks of the latter being similar to those of their white counterparts,29,30 suggesting either environmental factors acting on second-generation African-Caribbeans or gene-environment interactions. It has been suggested that an excess of pre- and perinatal obstetric complications in African-Caribbean mothers may be important, but again, there is no evidence to support this31; indeed, the contrary seems to be the case.32,33 Finally, the fact that rates are not markedly raised in the Caribbean suggests that genetic explanations alone are insufficient to account for the scale of these findings.
If social adversity, in a variety of forms, particularly if experienced for prolonged periods during early development, increases the risk of psychosis,34 then it may be that this is 1 contributory explanation for the high rates of psychosis among African-Caribbeans and other migrant groups. It is known, for example, that migrants are more likely to settle in urban centers and are subject to a greater degree of social adversity in general than their indigenous counterparts. Boydell and colleagues35 found that rates of schizophrenia were highest among African-Caribbeans when they lived in areas where they formed a relatively smaller proportion of the population; these findings hint at a possible role for social exclusion, discrimination, and isolation. Some further clues are offered by a small case-control study of first-episode schizophrenia in Camberwell and Ealing, London.36 This found that unemployment and long-term separation from parents before the age of 17 were particularly associated with an increased risk of schizophrenia in the African-Caribbean group.
The greater prevalence of disadvantage among migrant groups is a function of their structural position in society, a position that for many reflects institutionalized racism within host cultures. It has been proposed that individual-level racism is a contributory factor to the high rates.37 Direct evidence for this is limited, though a recent population-based prevalence study did suggest that experiences of discrimination may be linked to psychosis.38 For a more complete discussion, see the work of Sharpley et al.39
However, the evidence is still thin, and there is a clear need for further research to replicate and extend findings linking specific aspects of the social environment and risk of psychosis in migrant groups.
Understanding the causes of these raised rates, particularly in terms of investigating hypothesized associations with early development and with perceptions of disadvantage and discrimination, is methodologically challenging. Furthermore, it seems increasingly unlikely that any one factor, or even group of related factors, acts in isolation to “cause” psychosis in these population groups. How, therefore, can research in this area progress further to start to clarify the effects of potential risk factors?
One possible avenue is to concentrate either on factors that show a stronger association with psychosis in migrant groups than in indigenous populations or on those that have a comparatively greater prevalence in these groups, such as experience of discrimination and disadvantage.
Another potential avenue is to explore why some groups appear to be at lower risk than others. For example, the South Asian populations in the United Kingdom migrated in broadly the same era as the UK African-Caribbean population; they are exposed to similar levels of “urbanicity” and discrimination; and yet their risk appears to be markedly lower. Studying potential protective factors (such as strong social and family networks) may provide some clues that may help to clarify why some groups appear to be relatively more protected from psychosis than others.
Finally, although our knowledge of the role of such risk factors in the development of psychosis is progressing, it seems prudent at this stage to remain measured in both our interpretation and in the relative importance we assign to such findings. In this way, this field can move forward steadily and consistently. This will ultimately enrich our understanding of the causes of psychosis not only in migrant groups but also in the population as a whole.