Introduction
The finding of elevated incidence of schizophrenia amongst migrant and ethnic minority groups is well replicated; a recent meta-analysis of population-based studies, derived predominantly from the UK and continental Europe, indicated a 2.9 times [95% confidence interval (CI) 2.5–3.4] greater risk for schizophrenia in migrants relative to host populations (Cantor-Graae & Selten, Reference Cantor-Graae and Selten2005). This increase is particularly prominent among African-Caribbean migrants to the UK (Fearon et al. Reference Fearon, Kirkbride, Morgan, Dazzan, Morgan, Lloyd, Hutchinson, Tarrant, Lun Alan Fung, Holloway, Mallett, Harrison, Leff, Jones and Murray2006). Recent data from the Aetiology and Ethnicity in Schizophrenia and Other Psychoses (AESOP) study of first-onset cases of psychosis in the UK replicate findings from earlier studies (Harrison et al. Reference Harrison, Owens, Holton, Neilson and Boot1988, Reference Harrison, Glazebrook, Brewin, Cantwell, Dalkin, Fox, Jones and Medley1997; Bhugra et al. Reference Bhugra, Leff, Mallett, Der, Corridan and Rudge1997), in indicating incidence rate ratios (IRRs) for narrowly defined schizophrenia of 9.1 (95% CI 6.6–12.6) in African-Caribbean migrants relative to the white British population (Fearon et al. Reference Fearon, Kirkbride, Morgan, Dazzan, Morgan, Lloyd, Hutchinson, Tarrant, Lun Alan Fung, Holloway, Mallett, Harrison, Leff, Jones and Murray2006). AESOP study data additionally demonstrated an elevated IRR relative to the white British population among black migrants from sub-Saharan Africa (IRR 5.8, 95% CI 3.9–8.4; see also van Os et al. Reference van Os, Castle, Takei, Der and Murray1996), with more modest elevations apparent for various other ethnic minority groups, including migrants from other white backgrounds. Only amongst Asian (i.e. Pakistani, Indian and Bangladeshi) migrants was the IRR not significantly elevated (IRR 1.4, 95% CI 0.7–31), which is generally consistent with findings of minimal, if any, elevations in earlier incidence studies conducted within the UK (Bhugra et al. Reference Bhugra, Leff, Mallett, Der, Corridan and Rudge1997; Cantor-Graae & Selten, Reference Cantor-Graae and Selten2005).
The frequency of delusional or hallucinatory experiences in the general adult population greatly exceeds the prevalence of schizophrenia or other psychoses (van Os et al. Reference van Os, Hanssen, Bijl and Ravelli2000; Verdoux & van Os, Reference Verdoux and van Os2002). The prevalence of psychotic symptoms in the community also varies among migrant groups. For example, based on data from the Fourth National Survey of Ethnic Minorities in England and Wales, Johns et al. (Reference Johns, Nazroo, Bebbington and Kuipers2002) reported a 2.5-fold increase in hallucinatory experiences reported by individuals of African-Caribbean ethnicity relative to the white population, whilst hallucinatory experiences were half as common in south Asian as in white respondents. Similarly, general adult population data from the Ethnic Minority Psychiatric Illness Rates (EMPIRIC) study conducted in England (King et al. Reference King, Nazroo, Weich, McKenzie, Bhui, Karlsen, Stansfeld, Tyrer, Blanchard, Lloyd, McManus, Sproston and Erens2005) indicated a twofold increase in reports of psychotic symptoms by people of African-Caribbean origin relative to white British people, with non-significant elevations reported for Pakistani, Indian and white Irish groups and lower rates among the Bangladeshi group, relative to the white British group. Data obtained from the second National Survey of Psychiatric Morbidity in Great Britain indicated a somewhat greater than twofold elevation in reports of hallucinatory experiences among the black ethnic group, defined to include individuals of African-Caribbean and black African ethnicity, relative to white British persons (Johns et al. Reference Johns, Cannon, Singleton, Murray, Farrell, Brugha, Bebbington, Jenkins and Meltzer2004). Consistent with data from a Dutch general adult sample (van Os et al. Reference van Os, Hanssen, Bijl and Ravelli2000), these psychotic-like experiences (PLE) were observed to have similar demographic and clinical correlates to clinical psychosis (Johns et al. Reference Johns, Cannon, Singleton, Murray, Farrell, Brugha, Bebbington, Jenkins and Meltzer2004).
Differences in schizophrenia incidence rates and in the prevalence of psychotic-like experiences among minority groups of different ethnic origin provide an opportunity to identify factors that vary in concert with the incident rates and then to develop hypotheses about the aetiological mechanisms responsible for the differences. Establishing when during the course of development these differences appear and how they develop would inform both aetiology and prevention. The present study examined the associations of ethnicity and migrant status with putative antecedents of schizophrenia in a UK community sample of children aged between 9 and 12 years. Ethnicity and migrant status were related to the experience of a triad of putative antecedents of schizophrenia comprising (i) PLEs; (ii) a speech and/or motor developmental delay or abnormality; and (iii) a social, emotional or behavioural problem (for a full description of antecedent triad construction, see Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). The predictive validity of the antecedent triad for schizophrenia spectrum disorders has not yet been established. We recently argued, however, that using a combination of the childhood characteristics that have been reported to distinguish individuals who later developed schizophrenia is likely to afford a more sensitive and specific means of identifying children at risk for schizophrenia spectrum disorders than any antecedent alone (Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). Based on research conducted with adult samples, we hypothesized that first- and second-generation migrant children would report increased prevalence of PLEs and of the antecedent triad relative to non-migrant children. We further hypothesized that, relative to white British children, children of African-Caribbean ethnicity living in the UK would be characterized by increased prevalence of PLEs and experience of the antecedent triad, with other ethnic groups characterized by more modest increases in PLEs and triad prevalence.
Method
Sampling and recruitment
Detailed information regarding sampling and recruitment is provided in Laurens et al. (Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). Children aged 9–12 years and their primary caregivers were recruited via primary schools and general practitioners' (GP) practices located within three inner-city neighbourhoods in southeast London, England. The sample included the children and caregivers described previously (Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007), plus additional participants recruited via primary schools. The final sample included 2588 children (82% of eligible children) and 597 caregivers (19% of eligible caregivers). We previously established that children's data do not vary as a function of the availability of data from caregivers (Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007).
All families with a child aged 9–12 years who were registered with a collaborating GP practice received a letter from the practice inviting the child and caregiver to complete questionnaires independently and to return them via reply-paid mail. An audio-recording of the child's questionnaire on compact disc media accompanied the questionnaire in order to circumvent potential difficulties arising for children with poor reading ability. For children enrolled in school year levels 4, 5 and 6 at participating primary schools, the questionnaire was read aloud to the class by a research assistant who ensured that children responded independently. Each child was subsequently issued a questionnaire for their primary caregiver, who was asked to complete and return the questionnaire by reply-paid mail.
The present sample included the 595 participants (n=324, 55% female, mean age 10 years, 4 months) for which both child- and caregiver-reported data were obtained, with 127 (21%) participants recruited via GPs, and the remaining 468 (79%) via schools. Caregivers included mothers (86%), fathers (10.5%), grandparents (1.3%) or other (2.2%). As shown in Table 1, more than half of the children (66.3%) were members of an ethnic minority. Questionnaire return rates from each ethnic group were representative of the percentages of children within each ethnic group enrolled at participating schools (Department for Education and Skills, 2006). Of the children, 15.9% were born outside the UK, and more than half of the children (59.3%) had at least one parent born outside the UK.
Table 1. Percentages of children belonging to each ethnic group, and migrant status of the children within each ethnic group defined by parents’ and child's place of birth
Instruments
The full range of information assessed by questionnaires was described in Laurens et al. (Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). Only the measures used for the present analyses are described below.
Demographic information
Caregivers provided demographic information about the child, the place of birth (city and country) of the child and his/her parents, and indicated the child's ethnic group membership according to the 2001 UK Census ethnic categories defined by the Office of National Statistics (2001). Ethnic groups were subsequently collapsed into the following six categories: white British, white other, black African (including children of mixed black African and white heritage), African-Caribbean (including children of mixed African-Caribbean and white heritage), South Asian and Oriental (including children of mixed South Asian and white or mixed Oriental and white heritage), and other ethnicity (i.e. all other children not identified as belonging to one of the major ethnic groupings living in the UK, e.g. Latin American). South Asian and Oriental children were included within the same category because, although these groups constitute significant ethnic minorities within the UK, there were insufficient children from either ethnic group to examine these groups independently. Migrant status of the children was defined according to both parents' and child's place of birth within or outside of the UK. ‘Non-migrants’ were defined as children who were born in the UK who also had both parents born in the UK. ‘First-generation migrants’ were children who were born outside of the UK (these children all also had at least one parent born outside the UK). ‘Second-generation migrants’ were defined as children who had at least one parent with a foreign birthplace but who were themselves born within the UK. ‘Non-migrants’ also included UK-born children of UK-born parents who had a grandparent, or more distant ancestor, of foreign birth (i.e. ‘third-generation migrants’ or greater).
Psychotic-like experiences
Children answered nine questions assessing a range of PLEs. These items were revised and extended from five questions, derived originally from the Diagnostic Interview Schedule for Children (Costello et al. Reference Costello, Edelbrock, Kalas, Kessler and Klaric1982), that were used to assess self-reported hallucinatory experiences and delusional beliefs in children aged 11 years within the Dunedin Multidisciplinary Health and Development Study and shown to be predictive of later schizophreniform disorder (Poulton et al. Reference Poulton, Caspi, Moffitt, Cannon, Murray and Harrington2000). The questions were: (1) ‘Some people believe that their thoughts can be read. Have other people ever read your thoughts?’; (2) ‘Have you ever believed that you were being sent special messages through the television?’; (3) ‘Have you ever thought that you were being followed or spied upon?’; (4) ‘Have you ever heard voices that other people could not hear?’; (5) ‘Have you ever felt as though your body had been changed in some way that you could not understand?’; (6) ‘Have you ever felt that you were under the control of some special power?’; (7) ‘Have you ever known what another person was thinking even though that person wasn't speaking?’; (8) ‘Do you have any special powers that other people don't have?’; and (9) ‘Have you ever seen something or someone that other people could not see?’. Each PLE item was rated on a three-point scale: ‘0 – not true’; ‘1 – somewhat true’; or ‘2 – certainly true’. A dichotomous variable was computed to indicate the presence/absence of ‘certain experience’ of any PLE item (i.e. a score of ‘2’ on at least one of the nine items). Additionally, to provide an indication of the severity of children's experiences, a continuous variable was created by summing responses on the nine items to derive a total PLE score (range 0–18). Thus, children scoring highly on this continuous variable were characterized by a large number of ‘certain’ PLEs.
Triad of putative antecedents
Construction of the triad of putative antecedents of schizophrenia is described in detail in Laurens et al. (Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). Briefly, the triad encompassed three domains of putative childhood antecedents of schizophrenia: (1) caregiver-reports of any abnormality or developmental delay in speech and/or motor function; (2) caregiver-reports of ‘abnormal’ ratings (i.e. ratings within the top tenth percentile of UK population norms) of externalising (conduct problems, hyperactivity-inattention) and peer problems, and child-reports of emotional symptoms, on the Strengths and Difficulties Questionnaire (SDQ; Goodman, Reference Goodman1997; Goodman et al. Reference Goodman, Meltzer and Bailey2003); and (3) child-reports of certain experience of at least one PLE. A dichotomous variable was created to index the presence/absence of the antecedent triad (i.e. with difficulties in all three antecedent domains necessary for a rating of triad presence). A continuous variable indexing severity of children's problems within the antecedent triad was created by summing the standardized total scores obtained by children on each domain of the antecedent triad (i.e. total speech or/and motor abnormality Z score; total SDQ psychopathology Z score created as the sum of the scores obtained on the child-reported Emotional Symptoms subscale, and caregiver-reported Conduct Problems, Hyperactivity-Inattention, and Peer Problems subscales; and total PLE Z score). Hereafter, this variable is referred to as the ‘triad sum score’.
Statistical analyses
We conducted χ2 analyses to ascertain whether ethnicity and migrant status were related to the dichotomous variables coding the presence/absence of ‘certain experience’ of any PLE item and of the antecedent triad. Additionally, logistic regression analyses were employed to determine whether the presence/absence of PLEs and/or the antecedent triad varied by ethnic group or by migrant status, with associations expressed as odds ratios (OR). Ethnicity and migrant status were first entered individually to obtain unadjusted ORs, and then entered in a subsequent analysis to obtain the relative odds of experiencing at least one PLE and the antecedent triad when controlling for the interrelationship between the ethnicity and migrant status variables and for potential effects of sex of the child. ORs were computed for each ethnic minority group relative to the white British group, and for each migrant group relative to non-migrant children.
Analysis of variance was used to ascertain the effect of ethnicity (six levels: white British, white other, black African, African-Caribbean, South Asian and Oriental, and other ethnicity) and child's migrant status (three levels: non-migrant, first-generation migrant, second-generation migrant) on the continuous variables encoding the severity of difficulties experienced by the child (i.e. total PLE score and triad sum score). Planned comparisons were conducted between each ethnic minority group relative to the white British group, as well as comparing the two migrant groups and the non-migrant reference group. Post hoc Tukey tests were used to ascertain any total PLE score and triad sum score differences between the five ethnic minority (i.e. non-white British) groups.
Results
Psychotic-like experiences
The percentages of children within each ethnic group reporting ‘certain experience’ of at least one PLE item are reported in Table 2. The prevalence of reporting certain experience of at least one PLE item varied by children's ethnicity [χ2(5, n=590)=12.73, p=0.026]. However, migrant status was unrelated to PLE experience [χ2(2, n=529)=1.01, p=0.603].
Table 2. Percentages of children within each ethnic group who report certain experience of at least one PLE item and who experience all three domains comprising the antecedent triad, and mean total score for each ethnic group on continuous variables encoding the severity of children's experience of PLEs and the antecedent triad
PLE, Psychotic-like experience; s.d., standard deviation.
a The triad sum score was created as the sum of the standardized total scores obtained in each of the three domains of antecedent variables comprising the antecedent triad.
Logistic regression analyses were conducted to assess the associations between ethnicity and migrant status and child-reports of certain experience of at least one PLE. As presented in Table 3, relative to white British children, children of African-Caribbean ethnicity were significantly more likely to report certain experience of at least one PLE, whereas children of South Asian and Oriental ethnicity were half as likely to do so. After adjusting the ORs for potential interrelationships between ethnicity and migrant status and for the effect of child's sex, children of African-Caribbean ethnicity were almost twice as likely to report certain PLE experience as white British children. The reduction in risk apparent in South Asian and Oriental children relative to white British children remained at trend level only in the adjusted model. Migrant status was unrelated to certain experience of a PLE item in either the unadjusted or adjusted models.
Table 3. Associations of ethnicity and migrant status with certain experience of any PLE item and with experience of all three domains comprising the antecedent triad
PLE, Psychotic-like experience; OR, odds ratio; CI, confidence interval.
a Adjusted for the interrelationship between ethnicity and migration status, and for child's sex.
b Reference group.
c OR for the South Asian and Oriental group relative to the white British group was not computable, because no child in the South Asian or Oriental group satisfied criteria for the antecedent triad. These groups were instead compared using Fisher's exact test (for details, see text).
Mean scores on the continuous variable indexing severity of children's overall experience of psychotic-like phenomena, total PLE-score, are provided in Table 2. Children's ethnicity related significantly to total PLE score [F(5, 511)=3.09, p=0.009], whereas migrant status did not [F(2, 511)=0.73, p=0.480], and nor did ethnicity and migrant status interact [F(9, 511)=1.10, p=0.363]. Planned contrasts between each ethnic minority group and the white British group revealed that children of African-Caribbean ethnicity obtained higher total PLE scores than did white British children (p=0.037). Scores obtained by children in the black African, South Asian and Oriental, white other, and other ethnicity groups did not differ significantly from those obtained by white British children. Post hoc Tukey tests comparing total PLE scores obtained by children in the five ethnic minority groups revealed that total PLE scores in the African-Caribbean group were significantly elevated relative to scores within the South Asian and Oriental group (p=0.023). Planned contrasts between the two migrant groups and the non-migrant reference group were not significant.
Triad of putative antecedents
Table 2 provides the percentages of children within each ethnic group who displayed the triad of putative antecedents. The proportion of children meeting criteria for the triad of putative antecedents of schizophrenia varied by ethnicity [χ2(5, n=584)=10.89, p=0.054], whereas migrant status was again unrelated to triad presence [χ2(2, n=524)=0.58, p=0.748].
Logistic regression analyses (Table 3) indicated that, in addition to a significant relationship in the unadjusted model, after adjusting for the interrelationship between ethnicity and migrant status and for child's sex, African-Caribbean children were over three and a half times more likely to present the putative triad of antecedents than were children in the white British group. As no child in the South Asian and Oriental group met criteria for the presence of the antecedent triad, it was not possible to compute an OR for this group relative to the white British group. Instead, Fisher's exact test was employed to compare the prevalence of the antecedent triad in these two ethnic groups. This test revealed a trend for children of South Asian and Oriental ethnicity to be less likely to experience the antecedent triad than white British children (p=0.080, two-sided). Migrant status was unrelated to the experience of the antecedent triad, even after adjusting for the interrelationships between ethnicity and migrant status, and for child's sex.
Children's ethnicity was significantly related to the continuous variable reflecting the severity of children's problems within the antecedent triad, triad sum score [F(5, 514)=3.40, p=0.005], whereas migrant status was unrelated [F(2, 514)=0.52, p=0.595], and there was no interaction between ethnicity and migrant status [F(9, 511)=1.10, p=0.363]. Planned contrasts between each ethnic minority group and the white British group revealed that children of African-Caribbean ethnicity experienced greater triad sum score scores than did white British children (p=0.007), as did children of other ethnicity relative to white British children (p=0.015). Scores obtained by children in the black African and the South Asian and Oriental groups did not differ from those obtained by white British children. Post hoc Tukey tests were conducted to compare the triad sum score scores of the four ethnic minority groups. Results revealed no significant differences between ethnic minorities, although a trend for children of South Asian and Oriental ethnicity to experience lower scores than African-Caribbean children was observed (p=0.059).
Discussion
Ethnic group, but not migration status, was consistently associated with the putative antecedents of schizophrenia among 9- to 12-year-old children in our community sample. Children of African-Caribbean origin, as compared with white British children, presented elevated rates of certain experience of at least one PLE, higher total PLE scores, a greater prevalence of a triad of putative antecedents of schizophrenia and a higher total score for the antecedent triad. By contrast, there were trends for children of South Asian and Oriental origin, as compared with children of white British origin, to present lowered risk for the presence of certain experience of at least one PLE and of the antecedent triad. While no child of South Asian and Oriental ethnicity in the sample presented with the antecedent triad, each of the three components of the triad was experienced by at least one-quarter of children from this group. No interactions between ethnicity and migrant status were detected using any of the four indices of risk for schizophrenia. Further, no sex effects were observed.
The associations of ethnicity with the triad of putative antecedents of schizophrenia and with PLEs which we found among a community sample of 9- to 12-year-old children are similar to the associations observed in studies of adults in which the incidence of schizophrenia and the prevalence of psychotic symptoms in community samples vary by ethnic group. Notably, both in the present study of children and in the studies of adults living in the UK, it is individuals of African-Caribbean origin who show particularly pronounced vulnerability for schizophrenia and for the presence of psychotic symptoms in the absence of disorder (Johns et al. Reference Johns, Nazroo, Bebbington and Kuipers2002, Reference Johns, Cannon, Singleton, Murray, Farrell, Brugha, Bebbington, Jenkins and Meltzer2004; King et al. Reference King, Nazroo, Weich, McKenzie, Bhui, Karlsen, Stansfeld, Tyrer, Blanchard, Lloyd, McManus, Sproston and Erens2005; Fearon et al. Reference Fearon, Kirkbride, Morgan, Dazzan, Morgan, Lloyd, Hutchinson, Tarrant, Lun Alan Fung, Holloway, Mallett, Harrison, Leff, Jones and Murray2006). We have no comparative data regarding the rates of PLEs and the antecedent triad among African-Caribbean children (or South Asian and Oriental children) who reside in their countries of origin. The elevated incidence of schizophrenia observed in certain ethnic groups in Western Europe, however, is not explained by higher rates present in the countries of origin (Cantor-Graae & Selten, Reference Cantor-Graae and Selten2005).
The lack of evidence for elevated vulnerability among black African children contrasts with recent data indicating increased incidence of schizophrenia among this ethnic group relative to the white British population (Fearon et al. Reference Fearon, Kirkbride, Morgan, Dazzan, Morgan, Lloyd, Hutchinson, Tarrant, Lun Alan Fung, Holloway, Mallett, Harrison, Leff, Jones and Murray2006). Whether there is also an elevated prevalence of psychotic symptoms among black Africans in the general adult population is unclear, as previous studies have not typically distinguished black Africans, when included, from African-Caribbeans (Johns et al. Reference Johns, Cannon, Singleton, Murray, Farrell, Brugha, Bebbington, Jenkins and Meltzer2004). In the present study, children included within the black African group derived from diverse regions of sub-Saharan Africa, and were culturally and genetically heterogeneous. This heterogeneity may have obscured variation in vulnerability experienced by black African children deriving from different regions. Future research employing a larger sample of children from sub-Saharan Africa may elucidate such variation.
The sample did not include sufficient numbers to establish definitively whether children of South Asian and Oriental ethnicity present fewer antecedents than white British children. Clarifying this is important because it has implications for aetiology. If it is true that, among migrants, the prevalence of the putative antecedents of schizophrenia varies from higher than the local population to much lower, this would suggest that migrant status or minority group status per se is not associated with an increase in risk. Rather, it would suggest that certain ethnic groups present a vulnerability for schizophrenia that may be triggered or promoted by a foreign environment, while other ethnic groups display a profile of antecedents that is protective from the disorder. These ethnic-group differences in vulnerability, if replicated, present opportunities to identify both risk and protective processes that occur prior to illness onset.
Across all ethnic groups, a large proportion of children reported certain experience of at least one PLE. The prevalence of these experiences greatly exceeds the prevalence of schizophrenia and other psychoses in the general population. These data are consistent with findings from adult samples that suggest the existence of a symptomatic continuum between individuals in the general population and clinical cases of psychosis (van Os et al. Reference van Os, Hanssen, Bijl and Ravelli2000; Verdoux & van Os, Reference Verdoux and van Os2002). Whilst childhood self-reports of psychotic symptoms are relatively strongly, sensitively, and specifically related to later schizophreniform disorder (Poulton et al. Reference Poulton, Caspi, Moffitt, Cannon, Murray and Harrington2000), we have argued that using PLEs in combination with other childhood antecedents of schizophrenia will afford a more sensitive and specific means of identifying children at risk for schizophrenia spectrum disorders (Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007). The elevation in putative risk for schizophrenia apparent among African-Caribbean children relative to white British children (and arguably the potential risk reduction observed in South Asian and Oriental children relative to white British children) was more marked on indices of the antecedent triad than indices indexing only PLEs. The sensitivity and specificity of the antecedent triad for identifying those who will later develop schizophrenia is presently unknown, however, and will only be established by following children through the age period of risk for the disorder.
International replication of the observed ethnic-group differences in childhood vulnerability for schizophrenia will be necessary to establish the generalizability of the results outside the British context. Such research may be particularly informative if conducted within a setting in which minority ethnic group status can be dissociated from migration status, such as is possible within the USA. Using data from a birth cohort, Bresnahan et al. (Reference Bresnahan, Begg, Brown, Schaefer, Sohler, Insel, Vella and Susser2007) recently established substantially elevated rates of schizophrenia among African-Americans in comparison with white Americans. It would be interesting to determine whether African-American children are likewise characterized by an elevated vulnerability for schizophrenia relative to white American children using the risk indices developed within the current study.
The reliability of the study results may be affected by the relatively small sample of children recruited. The sample size may also have precluded detection of interaction effects between ethnicity and migration. Meta-analytic data indicate that migrant status is a risk factor for the development of schizophrenia spectrum disorders that varies by ethnicity (Cantor-Graae & Selten, Reference Cantor-Graae and Selten2005). Furthermore, the particularly elevated relative risk of schizophrenia apparent among second-generation migrants (4.5, 95% CI 1.5–13.1 v. 2.7, 95% CI 2.3–3.2 for first-generation migrants; Cantor-Graae & Selten, Reference Cantor-Graae and Selten2005) implies that processes acting proximally to migration do not, in isolation, explain the raised rates. That is, migrant status is a relatively crude proxy marker of risk for schizophrenia spectrum illnesses, and different migrant groups should not be regarded as deriving from a homogeneous population with a common effect size (Selten et al. Reference Selten, Cantor-Graae and Kahn2007).
If replicated, the findings present important opportunities for research aimed at identifying aetiological mechanisms that lead to schizophrenia and that protect against the disorder, and for testing the feasibility of preventive interventions during middle childhood. Prospective longitudinal investigations of children who experience the triad of putative antecedents (Laurens et al. Reference Laurens, Hodgins, Maughan, Murray, Rutter and Taylor2007) have the potential to identify processes by which genetic and pre- and perinatal factors interact with environmental factors during development to determine illness onset. These developmental processes may vary by ethnicity and thereby modify the prevalence of schizophrenia and of psychotic symptoms across ethnic groups.
Acknowledgements
The authors thank the children and caregivers who provided data, and the collaborating primary schools and GP practices for their assistance in conducting the research. We also thank Priscilla Lewis-Carter, Kate Akingbade, Delphine Theobald, Madi Ashdown, Husnara Khanom, Matthew Schroeder, Felicity Adams, Marzio Ascione, Kate Willis and David Carlos for assistance with data collection and/or entry. K.R.L. received a Young Investigator Award to present this work in poster form at the 11th International Congress on Schizophrenia Research in Colorado Springs, CO, USA (March 2007). She also gratefully acknowledges support from a UK Department of Health Postdoctoral Fellowship Award, a NARSAD Young Investigator Award, and the British Medical Association Margaret Temple Award for schizophrenia research. S.H. holds a Royal Society Wolfson Merit Award. All authors are affiliated with the NIHR Biomedical Research Centre, South London and Maudsley NHS Foundation Trust and Institute of Psychiatry, King's College London, UK.
Declaration of Interest
None.
