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Autism, ethnicity and maternal immigration

  • D. V. Keen (a1), F. D. Reid (a2) and D. Arnone (a3)
Abstract
Background

A growing number of European studies, particularly from Nordic countries, suggest an increased frequency of autism in children of immigrant parents. In contrast, North American studies tend to conclude that neither maternal ethnicity nor immigrant status are related to the rate of autism-spectrum disorders.

Aims

To examine the hypotheses that maternal ethnicity and/or immigration are linked to the rate of childhood autism-spectrum disorders.

Method

Retrospective case-note analysis of all 428 children diagnosed with autism-spectrum disorders presenting to the child development services in two centres during a 6-year period.

Results

Mothers born outside Europe had a significantly higher risk of having a child with an autism-spectrum disorder compared with those born in the UK, with the highest risk observed for the Caribbean group (relative risks (RRs) in the two centres: RR = 10.01, 95% CI 5.53–18.1 and RR = 8.89, 95% CI 5.08–15.5). Mothers of Black ethnicity had a significantly higher risk compared with White mothers (RR = 8.28, 95% CI 5.41–12.7 and RR = 3.84, 95% CI 2.93–5.02). Analysis of ethnicity and immigration factors together suggests the increased risk is predominately related to immigration.

Conclusions

Maternal immigration is associated with substantial increased risk of autism-spectrum disorders with differential risk according to different region of birth and possibly ethnicity.

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Copyright
Corresponding author
Correspondence: Daphne Keen, Consultant Neurodevelopmental Paediatrician, Room 2.35, 2nd Floor Clare House, St George's Hospital, Blackshaw Road, London SW17 0QT, UK. Email: daphne.keen@stgeorges.nhs.uk
Footnotes
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D.A. is currently supported by the Medical Research Council UK.

Declaration of interest

None.

Footnotes
References
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Autism, ethnicity and maternal immigration

  • D. V. Keen (a1), F. D. Reid (a2) and D. Arnone (a3)
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eLetters

Re: Re: Autism, ethnicity and maternal immigration

Daphne Keen, Consultant Neurodevelopmental Paediatrician
16 June 2010

We are grateful to Maenner and Durkin for their interest in this research (1), and for their helpful comments regarding future analyses.

We chose to use the mother rather than the child as the ‘unit of analysis’ for this study, because we were interested in the possible effect of a mother’s immigration status and ethnicity upon the risk of Autism Spectrum Disorder (ASD) in her children. Maenner and Durkin refer to an interesting study (2) which found a higher incidence of ASD among Somali children in Minnesota. That study focussed on the ethnicity of thechild as the risk factor, and therefore used the child as the unit of analysis with birth cohort data as a directly comparable denominator, thusavoiding some of the complications of our study.

We adjusted for differential family sizes between different ethnic groups, and were fortunate to find published estimated birth rates by ethnic group for London which had been calculated for 2002 (3), the mid-point of our study. Unfortunately similar birth rate information was not available by country of birth (i.e. immigration status), and this remains a limitation of our study which we acknowledge in the paper, as Maenner and Durkin mention.

Although these birth rates were for Greater London and not specific to the two boroughs in the study, we were able to make some borough-level adjustment. We weighted the birth rates for more detailed ethnic subgroups (Black Caribbean, Black African, Black Other; and Indian, Pakistani, Bangladeshi, Asian Other) according to the relative population of that subgroup in each borough, giving a more accurate birth rate for the higher level ethnic groups used in the study (Black and Asian). The overall borough-level birth rates were slightly higher in Lambeth than Wandsworth at that time (4), but this does not in itself represent a problem since the results of the study are relative risks within each borough.

The particular birth rate which we chose for the family size adjustment was the Total Period Fertility Rate (TPFR), partly because thishas an intuitive interpretation as the average family size (see Table 5 inoriginal paper, e.g. 2.3 children on average per mother of Black ethnicity)(1). However Maenner and Durkin are correct to point out that TPFRs are age-adjusted, and this has the potential to introduce some artefactual effects if the ethnic groups compared have different age distributions (within the child-bearing age range under study). We therefore ran the analysis again, this time adjusting for family size using the simpler General Fertility Rate (GFR), which was also available from the same source (2). The GFR is the number of live births in a year per 1000 women of child-bearing age. Reassuringly the results of this analysis are little changed from the first, and lead to the same conclusions (Table 1).

We wholeheartedly agree with Maenner and Durkin on the need to continue to explore ways to minimise potential sources of bias in future epidemiological studies of the effect of immigration on ASD.

1: Keen DV, Reid FD, Arnone D. Autism, ethnicity and maternal immigration. BJPsych 2010; 196: 274-81.

2: Minnesota Department of Health. Autism Spectrum Disorders Among Preschool Children Participating in the Minneapolis Public Schools Early Childhood Special Education Programs. Minnesota Department of Health. March 2009. (http://www.health.state.mn.us/ommh/projects/autism/report090331.pdf)

3: Klodawski E. Fertility of Ethnic Groups in London, 2002/3. DMAG Briefing 2204/24. Greater London Authority, 2004

4: Office for National Statistics, 2002 Vital Statistics, Table 4.1a(http://www.esds.ac.uk/Government/vitals/datasets/ons/2002.asp)

Table: Relative risk of autism-spectrum disorders, 1999-2005, by ethnicity and immigration status of mother   First analysis    Second analysis Ethnicity of mother and whether born in the UK or not Estimated average family size Relative risk vs White UK-born adjusted for family size a 95% CI for adjusted relative risk a    General Fertility Rate (GFR) b Relative risk vs White UK-born adjusted for GFR a 95% CI for adjusted relative risk a  Lambeth (n=137)               White 1.24       39.0     UK-born   1.00    ---     1.00    --- Immigrant   1.26 0.55 – 2.87     1.26 0.55 – 2.87 Missing               Black c   2.31       71.4     UK-born   2.07 1.14 – 3.75     2.10 1.16 – 3.82 Immigrant   8.21 4.94-13.63     8.35 5.03-13.87 Missing               Asian c 2.38       77.2     UK-born   1.01 0.23 – 4.33     0.98 0.23 – 4.20 Immigrant   5.52 2.57-11.83     5.35 2.49-11.46 Missing               Wandsworth (n=259)               White 1.24       39.0     UK-born   1.00    ---     1.00    --- Immigrant   0.73 0.49 - 1.08     0.73 0.49 - 1.08 Missing               Black c   2.30       70.6     UK-born   0.74 0.46 - 1.20     0.76 0.47 - 1.23 Immigrant   3.86 2.84 - 5.26     3.96 2.91 - 5.38 Missing               Asian c   2.30       73.6     UK-born   --- ---     --- --- Immigrant   1.13 0.67 – 1.90     1.11 0.66 – 1.86 Missing               Statistically significant results are shown in bold General Fertility Rate: the number of live births per annum per 1000 women of child-bearing age Including mixed Black/White or mixed Asian/White, respectively
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Conflict of interest: None Declared

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Re: Re: Autism, ethnicity and maternal immigration

Fiona D Reid, Senior Lecturer in Medical Statistics
14 June 2010

We are grateful to Maenner and Durkin for their interest in this research (1), and for their helpful comments regarding future analyses.

We chose to use the mother rather than the child as the ‘unit of analysis’ for this study, because we were interested in the possible effect of a mother’s immigration status and ethnicity upon the risk of Autism Spectrum Disorder

(ASD) in her children. Maenner and Durkin refer to an interesting study (2) which found a higher incidence of ASD among Somali children in Minnesota.

That study focussed on the ethnicity of the child as the risk factor, and therefore used the child as the unit of analysis with birth cohort data asa directly comparable denominator, thus avoiding some of the complications of our study.

We adjusted for differential family sizes between different ethnic groups, and were fortunate to find published estimated birth rates by ethnic group for

London which had been calculated for 2002 (3), the mid-point of our study.Unfortunately similar birth rate information was not available by country of birth (i.e. immigration status), and this remains a limitation of our study which we acknowledge in the paper, as Maenner and Durkin mention.

Although these birth rates were for Greater London and not specific to the two boroughs in the study, we were able to make some borough-level adjustment. We weighted the birth rates for more detailed ethnic subgroups (Black Caribbean, Black African, Black Other; and Indian, Pakistani, Bangladeshi, Asian Other) according to the relative population of that subgroup in each borough, giving a more accurate birth rate for the higher

level ethnic groups used in the study (Black and Asian). The overall borough-level birth rates were slightly higher in Lambeth than Wandsworth at that time (4), but this does not in itself represent a problem since the results of the study are relative risks within each borough.

The particular birth rate which we chose for the family size adjustment was the Total Period Fertility Rate (TPFR), partly because this has an intuitive interpretation as the average family size (see Table 5 in original paper, e.g. 2.3 children on average per mother of Black ethnicity)(1). However Maenner and Durkin are correct to point out that TPFRs are age-adjusted, and this has the potential to introduce some artefactual effects if the ethnic groups compared have different age distributions (within the child-bearing age range under study). We therefore ran the analysis again, this time adjusting for family size using the simpler General Fertility Rate (GFR), which was also

available from the same source (2). The GFR is the number of live births in a year per 1000 women of child-bearing age. Reassuringly the results of this analysis are little changed from the first, and lead to the same conclusions (Table 1).

We wholeheartedly agree with Maenner and Durkin on the need to continue to explore ways to minimise potential sources of bias in future epidemiological studies of the effect of immigration on ASD.

1: Keen DV, Reid FD, Arnone D. Autism, ethnicity and maternal immigration. BJPsych 2010; 196: 274-81.

2: Minnesota Department of Health. Autism Spectrum Disorders Among Preschool Children Participating in the Minneapolis Public Schools Early Childhood Special Education Programs. Minnesota Department of Health. March 2009. (http://www.health.state.mn.us/ommh/projects/autism/report090331.pdf)

3: Klodawski E. Fertility of Ethnic Groups in London, 2002/3. DMAG Briefing 2204/24. Greater London Authority, 2004

4: Office for National Statistics, 2002 Vital Statistics, Table 4.1a(http://www.esds.ac.uk/Government/vitals/datasets/ons/2002.asp)
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Conflict of interest: None Declared

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Re: Autism, ethnicity and maternal immigration

Matthew J. Maenner, Graduate Student
02 June 2010

The recent paper [1] by Keen, Reid and Arnone concluded there is an excess incidence of autism-spectrum disorders to British children of mothers born outside the UK, specifically those of African, Caribbean and Asian descent. An increased risk of autism to children of immigrant mothers has been observed elsewhere, including Somali schoolchildren in Minnesota, USA. [2]

One of the challenges of estimating risk among population subgroups is selecting an appropriate comparison group. In this analysis, the idealpopulation-based denominator would be all children born in the UK and residing in one of two London boroughs between 1999 and 2005. As often the case in epidemiological studies, this population is not readily identifiable; the authors needed to construct a denominator using available data sources.

From the 2001 Census, the authors identified their comparison group as all women of childbearing age living in Lambeth or Wandsworth. Becausethe risk of having a child with autism is contingent upon the number of children one has, the analysis controlled for age-adjusted fertility ratesfor each ethnic group. While this approach recognizes that not all women are mothers and some have more children than others, it requires other assumptions that may affect interpretation of the results.

First, the average number of children per mother by ethnicity assumesforeign-born mothers have the same number of children as their UK-born counterparts. If foreign-born mothers have, on average, more children thanUK-born mothers of the same ethnicity, the autism rate would be overestimated in the foreign-born group and underestimated in the UK-born group. The authors noted their fertility statistics were only available byethnicity and not by maternal country of birth.

Second, in using ethnic-specific fertility rates for all of Greater London, the authors have assumed these fertility rates are consistent across boroughs. As total fertility rates are age-adjusted, they do not necessarily correlate with the actual number of births occurring in a place. If the population in Wandsworth has a higher proportion of women of child-bearing age [3] than the overall region, the total fertility ratemay underestimate the actual birth counts in Wandsworth over a given period. This could contribute to the finding that the autism rate among children of UK-born white women was more than 5 times higher in Wandsworththan Lambeth. (Table 5) [1]

Further analysis based on empirical birth data would eliminate some of the questions surrounding the results of the present analysis. At least a portion of the striking excess risk of autism in children of immigrants reported in this study may be due to under-estimation of the number of minority children of foreign-born mothers, or other statistical artifacts. The authors’ efforts to incorporate multiple data sources is admirable, but an analysis using actual (rather than theoretical) birth counts and restricting cases and controls to consistent birth years would greatly strengthen the evidence provided by this study. Given the international relevance and sensitivity of this topic, we hope the authorsconsider these possibilities and continue to explore ways to minimize potential sources of error and bias in studies related to the mental health of children of immigrants.

Matthew J Maenner & Maureen S Durkin

References1. Keen DV, Reid FD, Arnone D. Autism, ethnicity and maternal immigration. Br J Psychiatry 2010; 196: 274-81.

2. Minnesota Department of Health. Autism Spectrum Disorders Among Preschool Children Participating in the Minneapolis Public Schools Early Childhood Special Education Programs. Minnesota Department of Health. March 2009. (http://www.health.state.mn.us/ommh/projects/autism/report090331.pdf)

3. Wandsworth Guardian. Statistics prove Wandsworth Nappy Valley has highest birth rate. Wandsworth Guardian. June 2009.(www.yourlocalguardian.co.uk/news/4439329.Statistics_prove_Nappy_Valley_has_highest_birth_rate/)

We have no conflicts of interest to declare.

Contact information:

Matthew J Maenner, PhD Candidate in Population Health Sciences, University of Wisconsin School of Medicine and Public HealthWaisman Center Rm A1211500 Highland AveMadison, WI 53705USA

Maureen S Durkin, PhD, DrPH,Professor of Population Health Sciences and PediatricsUniversity of Wisconsin School of Medicine and Public HealthWarf Building Rm 789610 S Walnut StMadison, WI 53705USA
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Conflict of interest: None Declared

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