Environmental Factors in Schizophrenia: The Role of Migrant Studies
Paul Fearon1,2and Craig Morgan2
2Division of Psychological Medicine, Institute of Psychiatry,
King’s College London
There is now compelling evidence that migrant groups in
phrenia and other psychotic disorders. Though the findings
of earlier studies were greeted with skepticism, and as-
cribed 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 in-
teract to increase the risk of psychosis, both in migrant
groups and in the general population.
Key words: ethnicity/environmental/incidence/psychosis/
What Is the Evidence That Migrant Groups Have
a Higher Incidence of Schizophrenia?
Early Studies: The United States
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’s1landmark
study of Norwegian migrants to the United States dem-
onstrated a 2-fold increase in first admission rates for
schizophrenia compared with native-born Americans or
first-admission rates for schizophrenia among the foreign-
born residents of New York State, taking account of
differing population age structures and urbanization.
Later Studies: The United Kingdom
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 popula-
tions reported higher than expected rates in migrants
of African-Caribbean origin.4,5There 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
of nonstandardized diagnoses, and uncertainty over the
completeness of case ascertainment).
However, many of these early methodological prob-
lems have since been overcome. The seminal study by
Harrison et al.6in Nottingham was the first study of
its kind that was based on sound epidemiological princi-
ples, including prospective case finding within a defined
catchment area, standardized assessments of mental
state, operational diagnostic criteria, and extensive use
of collateral history.7They found that the incidence
rate of schizophrenia was raised more than 12-fold in
the African-Caribbean community, compared with the
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
eral population. Three of these studies employed a pro-
spective design and ascertained cases by first contact of
both employed a similar methodology for case ascertain-
ment and assessment as the World Health Organization
10-country study.10The incidence rates of schizophrenia
in these studies were raised in the African-Caribbean
population; even continuing uncertainties in the de-
nominator could not account for the findings. King
and colleagues11found 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,12employing a case register ap-
proach, found increased rates of schizophrenia for
both African-Caribbeans and black Africans (ie, people
1To whom correspondence should be addressed; e-mail:
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of sub-Saharan African origin, a more recent migrant
group to the UK).
Latest Studies: The United Kingdom and Europe
has come from a number of studies performed in other
countries in Europe on other migrant groups. These sug-
gest 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
Morocco13,14and 1 study of immigrants to Sweden from
East Africa15demonstrate a higher incidence of schizo-
phrenia in these groups.
The UK-based AESOP study,16conducted 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
ing 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.
Are the Rates of Other Psychoses Raised?
Although the majority of such studies have concentrated
on schizophrenia, some studies have examined the rates
of other psychoses in migrant groups. Overall, the evi-
dence points to the rates for other psychoses being raised.
Leff et al.,17Bebbington et al.,18and Hunt et al.19all
ulation in the UK. A middle syndrome between this and
schizophrenia (schizomania) has also been reported as
more common.20Furthermore, Selten and colleagues14
ribbean migrants to the Netherlands. Selten and col-
leagues21also 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 ill-
ness 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,
nia among African-Caribbeans and black Africans com-
pared 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
Is the Incidence of Schizophrenia and Other Psychoses
Raised Equally in All Migrant Groups?
In their recent meta-analysis of population-based inci-
dence studies of schizophrenia in migrant populations,
Cantor-Graae and Selten23found a mean weighted re-
lative risk (RR) for developing schizophrenia among
migrant groups of 2.9 compared with indigenous popula-
tions. This effect was more marked in migrants from ei-
ther 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 (per-
sonal 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 schizophre-
nia in the Asian population in London, but they did note
elevated rates in those over 30 years old. King and cow-
orkers11found 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. Understand-
ing differences between groups who migrated to the UK
during the same era, and who appear to experience sim-
ilar levels of racial discrimination and social disadvan-
tage, may reveal important factors that play a part in
the raised rates of psychosis in certain ethnic minority
Is the Rate of Schizophrenia Raised in the Country
The obvious question from the above findings is whether
these raised rates exist in the countries of origin of mi-
grant 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,24Trinidad,25and Barbados,263 is-
lands 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
populationin theUnitedKingdomand weresignificantly
lower than the comparable rate for the UK African-
Caribbean population. Selten et al.27further found that
incidence rates in the Surinam were significantly lower
than among Surinamese migrants to the Netherlands.
P. Fearon & C. Morgan
Why Are Incidence Rates Raised in Migrant Groups?
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?
Ødegaard1had suggested that a shared predisposition
to migration and to schizophrenia might account for the
excess of schizophrenia noted in Norwegian migrants in
of schizophrenia in Surinamese migrants in the Nether-
lands, combined with the fact that about half the inhab-
itants of Surinam migrated to the Netherlands, makes
selective migration an unlikely explanation.28Further,
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 associa-
tion with migration and onset5and by the fact that rates
are also elevated in the second generation.
The Role of Genetic or Neurodevelopmental Factors
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 counter-
parts,29,30suggesting 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.32,33Finally, the fact
that rates are not markedly raised in the Caribbean sug-
gests 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 develop-
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
degree of social adversity in general than their indigenous
counterparts. Boydell and colleagues35found 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
hintat apossible roleforsocialexclusion,discrimination,
and isolation. Some further clues are offered by a small
case-control study of first-episode schizophrenia in Cam-
berwell and Ealing, London.36This found that un-
employment and long-term separation from parents
before the age of 17 were particularly associated with an
increased risk of schizophrenia in the African-Caribbean
groups is a function of their structural position in society,
a position that for many reflects institutionalized racism
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 ex-
periences 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, particu-
larly in terms of investigating hypothesized associations
with early development and with perceptions of dis-
advantage and discrimination, is methodologically chal-
lenging. 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 pro-
gress further to start to clarify the effects of potential
One possible avenue is to concentrate either on factors
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 exam-
ple, 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 protec-
tive 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 progress-
ing, 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 en-
rich our understanding of the causes of psychosis not
only in migrant groups but also in the population as
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