Tracking down the agent(s) responsible for the elusive “migrant” effect in schizophrenia bears similarities with the clever plot twists in a well-crafted crime novel. The new study by Jeremy Coid and coworkers makes substantial headway toward narrowing down the list of suspects, with the spotlight increasingly focused on ethnicity. The current venture has a number of outstanding strengths: large sample size, robust denominator data, and stringent methods of case ascertainment, including leakage analysis. The choice of venue of East London, an area characterized by socioeconomic deprivation, is a strategic advantage, in that the effect of ethnicity can be teased out from socioeconomic disadvantage. The findings indicate that ethnicity had a stronger effect on risk magnitude than did generational status (i.e., place of birth). Black Caribbeans were the only ethnic group where generational status “mattered,” an effect that the authors attribute to differences in the age structures of the underlying populations at risk.

How best to interpret these results, and where do they lead us?
Coid and colleagues argue that the cumulative effect of the various “pressures” operating on migrants might be roughly similar across generations, with some exceptions. In this case, the exceptions are all the more interesting, and likely to be the most informative. The fact that second-generation black Caribbeans had markedly higher rates of psychoses than black Caribbeans born in the Caribbean suggests differential pressures operating on these two groups. Although these pressures have yet to be identified (more detective work needed!), some mechanisms suggest themselves. The generation of black Caribbeans that emigrated to the U.K. during the 1950s and 1960s were more readily incorporated into the labor market than today’s generation (second-generation immigrants). Although their jobs might have primarily been in the service or unskilled sectors, the ability to get a “foot” into society and earn a living could have represented substantial gains for this generation in terms of positive self-esteem and protection against feelings of negative self-worth and/or outsider status. In contrast, the second generation of African-Caribbean immigrants seems to have more difficulty securing a place within U.K. society. Their difficulties may be compounded by unstable home environments. Ethnic groups with strong familial ties, such as Asians or the example of the Turks in The Netherlands (Selten et al., 2001), appear better able to buffer the negative effects of discrimination and marginalization. Thus, the extent to which families are able to provide stable home environments seems to be a crucial modifier of minority group “stress,” and we may very well see a resurgent interest in the near future in studies focusing on the schizophrenogenic family.

It may be noted that Coid and coworkers’ results are well in line with the social defeat hypothesis (Cantor-Graae and Selten, 2005; Selten and Cantor-Graae, 2005; Selten et al., 2007), which suggests that a key contributor to the migrant psychosis phenomenon is long-term chronic exposure to social defeat, possibly resulting in dopamine disturbances in the brain. While discrimination may play a key role in the experience of social defeat, the mechanisms that enhance vulnerability to psychosis in any specific minority group are probably complex. Certainly there is a need for confirmatory studies from settings where waves of migration span several generations, for example, countries such as France and Spain, in order to re-test Coid and colleagues’ findings. However, results from Israel, a country with a long history of heterogeneous migration, confirm the notion that “outsider” status may vary from group to group (Weiser et al., 2008) and across generations (Corcoran et al., 2008) and that this may in turn determine that group’s risk for schizophrenia. In other words, context may well prove to be “everything.” Thus, investigations that can shed further light on the elements that make up the contextual environment are likely to be especially fruitful.

The broader implications of Coid and coworkers’ important new migrant study are several. We need to once and for all accept the notion that schizophrenia is not uniformly distributed among groups, and to intensify research efforts that may further elucidate the nature of the underlying exposure variable(s). Although it may take considerable effort to track down the “culprit,” the good news is that social factors lend themselves to preventative interventions. There may well be a glimpse of light at the end of the tunnel.

References:

Selten JP, Veen N, Feller W, Blom JD, Schols D, Camoenië W, Oolders J, van der Velden M, Hoek HW, Rivero VM, van der Graaf Y, Kahn R. Incidence of psychotic disorders in immigrant groups to The Netherlands. Br J Psychiatry. 2001 Apr 1;178():367-72. Abstract

Cantor-Graae E, Selten JP. Schizophrenia and migration: a meta-analysis and review. Am J Psychiatry. 2005 Jan 1;162(1):12-24. Abstract

Selten JP, Cantor-Graae E. Social defeat: risk factor for schizophrenia? Br J Psychiatry. 2005 Aug 1;187():101-2. Abstract

Selten JP, Cantor-Graae E, Kahn RS. Migration and schizophrenia. Curr Opin Psychiatry. 2007 Mar 1;20(2):111-5. Abstract

Weiser M, Werbeloff N, Vishna T, Yoffe R, Lubin G, Shmushkevitch M, Davidson M. Elaboration on immigration and risk for schizophrenia. Psychol Med. 2008 Aug 1;38(8):1113-9. Abstract

Corcoran C, Perrin M, Harlap S, Deutsch L, Fennig S, Manor O, Nahon D, Kimhy D, Malaspina D, Susser E. Incidence of Schizophrenia Among Second-Generation Immigrants in the Jerusalem Perinatal Cohort. Schizophr Bull. 2008 Jul 22; Abstract

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