21 November 2008. Ever since Norwegian psychiatrist Örnulv Ödegaard found a heightened risk of schizophrenia in Norwegians who migrated to the United States (Ödegaard, 1932), researchers have been trying to understand why some immigrants seem more prone to develop schizophrenia than natives of their chosen land. To untangle the effects of migration and ethnicity, a research team led by Jeremy Coid of St. Bartholomew’s Hospital in London, England, assessed the incidence of psychosis in first- and second-generation migrant groups in the United Kingdom. As reported in the November Archives of General Psychiatry, their study turned up little evidence of generational differences in any ethnic group, except for black Caribbeans. In that community, the second generation seemed more likely than the first to develop nonaffective psychoses.
Coid’s collaborators included two researchers from the earlier population-based Ætiology and Ethnicity in Schizophrenia and Other Psychoses (ÆSOP) study (Kirkbride et al., 2006). Conducted in England, it had found that the incidence of schizophrenia and other psychoses varied with age, sex, ethnicity, and location. The ÆSOP researchers, led by James Kirkbride of the University of Cambridge, England, saw these variations as evidence that genes and environment together shape psychosis risk. The results strongly countered the notion that schizophrenia rates remain constant no matter where in the world you look.
Consistent with prior research on schizophrenia and migration (for reviews, see Cantor-Graae and Selten, 2005; McGrath et al., 2004), the ÆSOP study found a three times higher risk of psychosis in minority ethnic individuals than in white British subjects after adjusting for age, sex, and location. Whether the increased risk in immigrants extends to, or even grows, in their offspring, remains an open question. Coid and associates wondered whether generational effects would differ by ethnicity, gender, or type of psychosis.
Tracking cases in East London
In the East London First Episode Psychosis Study, Coid and coworkers at St. Bartholomew’s Hospital, with their collaborators at the University of Cambridge, studied an ethnically diverse population that lived in poverty-stricken boroughs of East London. The researchers took extra care to identify everyone from 18 to 64 years old who contacted mental health services during a two-year period about a first episode of likely psychotic illness. Willing subjects who had been diagnosed with a psychotic disorder underwent testing, and two researchers used the available clinical and test-based data to arrive at diagnoses by consensus.
A multiethnic panel of researchers coded ethnicity, based on subjects’ self-reported ethnicity, their place of birth, and their parents’ birthplaces. With 484 patients, the study could examine psychosis risk in six ethnic groups: black Caribbean, black African, Asian (Indian, Pakistani, and Bangladeshi), white British, white other (mostly Irish and European), and other (Chinese, other Asian, and mixed ethnicity).
Using the incidence rate ratio (IRR)—quite simply, one incidence rate divided by another—Coid and colleagues compared the risk in various ethnic groups with that in the reference group of white British who were born in the U.K. They separately analyzed the risk of nonaffective and affective psychoses, while controlling for age and sex. Nonaffective psychoses included schizophrenia, schizophreniform disorder, and schizoaffective disorder; affective psychoses consisted of bipolar, depressive, and other psychotic mood disorders.
Talkin’ ’bout which generation?
Turning first to nonaffective psychoses, the investigators found an elevated incidence in all of the migrant groups except the “other” group. Compared to white British individuals, black Caribbeans, the most at-risk group, showed a fourfold increase (IRR, 4.2; 95 percent CI, 3.0-5.8) and black Africans a threefold increase in risk (IRR, 3.4; 95 percent CI, 2.4-4.7). The Asian and white other groups showed smaller, but significant, increases in risk. Previous studies have likewise found a high rate of psychosis in black Caribbean and black African populations in Europe.
Pursuing hints in the data that generational status might be linked to the risk of nonaffective psychoses, Coid and associates found higher rates in the offspring of black Caribbean immigrants than in black individuals who had migrated from the Caribbean themselves (IRR, 1.6; 95 percent CI, 1.1-2.4; P = .02). They ascribe the generational differences to age: most of the second-generation black Caribbeans were less than 44 years old, whereas the preceding generation, which mainly arrived in the U.K. during the 1950s and 1960s, tended to be over 35. In short, the second generation tended to be in the age range at which psychoses typically emerge. The relatively small overlap between the two cohorts’ ages rendered statistical controls for age “somewhat ineffective,” Kirkbride told SRF via e-mail. In the 35-44 age range, where there was a sufficient population of both first- and second-generation black Caribbean immigrants to obtain precise rates, incidence of psychoses was similar between generations, who were both at equally raised rates compared with the white British.
Separate analyses for men and women revealed significant elevations only in Asian women. Notably, the first generation showed a nearly fourfold risk increase in rates of nonaffective psychoses relative to native British white women (IRR, 3.6; 95 percent CI, 2.1-6.4).
As for affective psychoses, the study again found a four times higher risk in black Caribbeans (IRR, 4.0; 95 percent CI, 2.4-6.9) and a three times higher risk in black Africans (IRR, 2.7; 95 percent CI, 1.5-4.9) than in the white British group. In addition, the white other group showed twice the risk of the reference group (IRR, 2.3; 95 percent CI, 1.3-4.0). Contrary to nonaffective psychoses, however, the Asians showed no reliable increase in the relative risk of affective psychoses, nor did generation status and ethnicity interact in any of the groups. Coid and associates point to the cohesiveness of the Asian immigrant community as a factor that may bolster them against discrimination and other tribulations that might otherwise make them vulnerable, though sex differences may remain.
A raft of possible explanations
The researchers view their findings as support for environmental explanations for the increased risk of psychotic illness in migrant groups (see SRF live discussion). Being the new kid on the block can be hard enough; moving to a different culture in a new land might bring out a predisposition to psychosis, according to the stress-vulnerability model. Furthermore, some researchers argue that the prolonged experience of “social defeat,” a consequence of second-class or outsider status, might increase schizophrenia risk via the mesolimbic dopamine system (see Selten and Cantor-Graae, 2007). “That the excess risk of psychoses for Asian immigrants in our sample appeared to be restricted to women provides anecdotal support for the social defeat hypothesis given the additional pressure of marginal status faced by some women in Indian, Pakistani, and Bangladeshi communities,” Coid and colleagues write.
Family structure might also play a role. In their homeland, Caribbeans tend to receive support from an extended family, but the high percentage of black Caribbean families headed by single parents in the U.K. suggests that migration frays these ties. Children who remain separated from a parent for a long time seem to be more susceptible to psychosis.
As for other possible explanations, Coid and colleagues doubt that either social class or cannabis use can account for their findings. They had previously shown that individual differences in social class cannot account for ethnic differences in psychosis rates in this sample, and European studies suggest that the black Caribbean population uses cannabis no more than the general population does.
The East London study did not explicitly test any of these explanations, so the investigators can only connect the dots between their findings and those that came before. Another generation of studies will have to discover whether the conclusions extend to other peoples moving to and from other places, particularly developing nations.—Victoria L. Wilcox.
Coid, JW, Kirkbride JB, Barker D, Cowden F, Stamps R, Yang M, Jones PB. Raised incidence rates of all psychoses among migrant groups. Findings from the East London First Episode Psychosis Study. Arch Gen Psychiatry. 2008 Nov;65(11):1250-1258. Abstract