Submitted:
26 June 2026
Posted:
09 July 2026
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Abstract
Background: The increase in prevalence of autistic spectrum disorder (ASD) in children in most countries of the world has been attributed to assortative mating meaning that spouses have found each other guided by features of ASD leading to increased genetic predisposition in the offspring. This does not explain the differences in prevalence of ASD in subgroups of the population with similar mating patterns. This study tested the hypothesis that parental genetic heterogeneity is associated with ASD in the offspring. Methods: Included in this study were data from population based studies reporting number of patients with ASD in populations with known high, moderate or low genetic heterogeneity. The prevalence (per 10000 of population) was compared between groups. Compared were the percentages of children with learning difficulties in the three groups. Results: The prevalence of ASD across spouse pairs of high, moderate or low genetic heterogeneity for 182016 children with ASD in childhood populations totalling 62 290 632 children in the USA, the United Kingdom, Sweden, Israel and Australia were compared. In all studies reporting data prevalence of ASD was significantly higher in offspring of parents with high genetic heterogeneity ranging from 12.9 to 280.5 compared to offspring of parents with moderate genetic heterogeneity with a range of 9.0 to 243.1 and low genetic heterogeneity with a range of 4.6 to 71.7. The percentage of children with learning difficulties showed no consistent difference between groups. Conclusion: Autism spectrum disorder in offspring is associated with parental genetic heterogeneity.
Keywords:
autism
; prevalence
; assortative mating
; genetics
; gene expression
; outbreeding
; inbreeding
Key points
Question: Is the degree of parental genetic heterogeneity associated with prevalence of autistic spectrum disorder (ASD) in offspring?
Findings: In a comparative observational study including 182016 children with ASD in childhood populations totalling 62290632 it was found that in the USA, Canada, the United Kingdom, Sweden, Israel and Australia comparing subgroups of spouses of high, moderate and low genetic heterogeneity ASD prevalence in the offspring was higher with higher genetic heterogeneity.
Meaning: Higher parental genetic heterogeneity is associated with increased prevalence of ASD in the offspring.
Introduction
Background
Previous studies attributed differences in prevalence of autism spectrum disorder (ASD) in different ethnic groups within a population to causes including health care seeking behaviour leading to under reporting, perception of ASD as an abnormality, maternal stress and other maternal factors including consequences of metabolic syndrome in pregnancy [1,2,3,4]. This does that despite overall lower socioeconomic status in the African American community in the USA autism prevalence has remained the higher in this population group for more than 20 years than in the more affluent Caucasian or Asian communities and this also applies to the data in the United Kingdom (see below) and consistently lowest in Native American populations with low socioeconomic status. ASD as evident from studies on monozygotic twins is inherited to more than 90% [5].
It is therefore very likely that the cause for differences in prevalence between subgroups of a population is genetic.
In a recent systematic review [6] we found that pooled correlation coefficients comparing Social responsiveness scale scores (SRSS) reflecting ASD features between spouses for families with and without offspring with ASD were significantly different. The correlation was lower for spouses with ASD in the offspring. As ASD features have a genetic background the hypothesis is presented that the differences in genetically determined SRSS is due to genetic heterogeneity of parents increasing the risk of ASD in the offspring.
Objectives
The objectives of this investigations were therefore:
To investigate whether parental genetic heterogeneity is associated with ASD prevalence in the offspring.
To investigate whether learning difficulties in offspring with ASD is associated with parental genetic heterogeneity.
Methods
Study Design
The study was designed as a retrospective observational study comparing groups following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement [7] (See Appendix A).
Setting: The data were extracted from published databases used in previous studies.
Participants
Eligibility criteria:
Definitions of included groups:
Parental genetic heterogeneity was defined as known locus heterogeneity where single nucleotide polymorphisms (SNP) in parents indicated genetic heterogeneity.
Included were in addition data from population groups which did not have studies investigating genetic heterogeneity by genome analysis but where anthropological studies have defined the degree of genetic heterogeneity based on studies of mating behaviour.
High parental genetic heterogeneity: This was defined as evident from previous genetic studies and included spouse pairs with African origin or different ethnic origin[8].
Moderate parental genetic heterogeneity:
This included spouse pairs with Caucasian or Asian ancestry[8] .
Low parental genetic heterogeneity:
This included groups with known low genetic heterogeneity including Native American populations in the USA, Irish traveller and Roma communities in the United Kingdom, ultraorthodox Jewish and Israeli Arab communities in Israel and Australian aborigines groups. In Australian aborigines groups even though genetic studies are lacking there have been strict mating rules observed avoiding intrafamiliar unions but limiting unions to within tribal groups which are known to have evolved in isolation from other remote Australian aborigines groups for thousands of years[9,10,11,12,13].
The prevalence of ASD and percentage of children with learning difficulties in the group with ASD in children of the above subgroups of populations was investigated and compared.
Inclusion criteria:
Data were only extracted from population based studies reporting the number of children with ASD and the number of children in a general reference population within the same country. In addition studies were reported where the raw data were not available but summary statistics for groups with known degree of parental genetic heterogeneity.
Exclusion criteria:
Case control studies and cohort studies with control group were excluded. Excluded were also studies where it was not possible to extract data for a population with defined genetic heterogeneity.
Study characteristics were analysed separately.
Sources of bias: Bias from differences in autism prevalence because of differences in time of study taking into account the increasing prevalence of ASD over time were addressed by separate analysis of raw data for each study separating data from investigations of ASD prevalence at different time points.
Data were presented separately for each country to avoid bias from the prevalence of arranged marriages, which by a reduction of assortative mating was previously shown to reduce the prevalence of autism[6] .
Statistical methods
Prevalence was calculated as per 10000 of the population and learning difficulties as percentage of children with ASD. Prevalence rates and percentages with learning difficulties were compared using chi-square test. In addition to reporting the p-value with a chosen probability of erroneous rejection of the null hypothesis of <0.05, odds ratios with 95% confidence interval were reported and for risk of learning difficulties the Risk Ratio with 95% confidence interval. The statistical software used was Epi InfoTM 7.1.3.10 (Centers for Disease Control and Prevention (CDC), Atlanta, USA).
Study size: As there were no previous studies on the relationship of parental genetic heterogeneity and autism prevalence no sample size calculation could be performed.
Results
Characteristics of Included Studies
Participants: Eight studies qualified for inclusion: One from the United Kingdom, two from the USA, one from Sweden, one from Ireland, one from Israel (See Table 1) one from Canada and one from Australia. The prevalence of ASD across spouse pairs of high, moderate or low genetic heterogeneity for 182016 children with ASD in childhood populations totalling 62290632 children were compared.
Study characteristics
For England the most recently reported nationwide study reporting on ethnic group specific data was used[14]. The study used as population data from the National Pupil Database (NPD) of England. The NPD is a total school population registry collected in England
By the Department for Education that maintains counts of all pupils aged 2 to 21 years under State education provision. From this data base the school spring census of the year 2017 was used. Children with a formal diagnosis of autism spectrum disorder were identified from the NPD based Special Educational Needs and Disability (SEND) registry. Children with autism and learning difficulties were identified from the same data base.
The first study systematically analysing autism prevalence in different ethnic group nationwide was the study by Dyches et al in the USA[15]. It was population based and included as the total number of children aged 5–19 years and for each ethnic group data from the U.S. Census report for the year 2000. The number of children with autism was taken from data reported under the Individuals with Disabilities Education Act of 1997 (IDEA ’97) (United States Department of Education, 2002, 2001) (IDEA). Because the IDEA data reported the number of children with autism ages 6–21 years to make the data comparable, it was necessary to multiply the IDEA autism data by 15/16ths (.9375) in each ethnic category.
Subsequently the Autism and Developmental Disabilities Monitoring (ADDM) Network which provides active surveillance of the prevalence of Autism spectrum disorder across multiple sites in the USA provided reports for 8 year old children. A report which allowed calculation of prevalence in offspring of highly genetically heterogeneous parents and moderately heterogenous parents had reported data from 11 ADDM Network sites across the USA from 2020[16].
One study from Sweden analysed the prevalence of autism and Asperger syndrome in children born in Malmoe between 1980 and 2005. Data were obtained from the Malmoe child psychiatric clinic with a diagnosis resulting from a multidisciplinary approach. The majority of children (75%) was between the ages of 8 to 15 years. Prevalence data could be obtained by comparison with the Swedish Medical Birth Registry of all individuals born in Malmoe during the study period. The data allowed comparison of prevalence in the genetically highly heterogeneous parental group from Sub-Saharan Africa and the moderately genetically heterogeneous group including Sweden, other Nordic countries, Western Europe/USA, previous Eastern Europe and East Asia combined[17]. In a retrospective analysis of case records of 366 children presenting to the paediatric developmental service in the National Children’s Hospital in Tallaght, Ireland between 2007 and 2009 presented data on the percentage of children with moderate to severe cognitive disability in patients from Africa compared to Ireland. The data did not contain information about a reference population so prevalence data were unavailable[18].
In the population based study in Israel data were derived from the Israeli National Insurance data base, which is a welfare governmental organisation responsible for the social security of the residents of Israel. The recorded diagnoses of ASD were based on assessments by a multidisciplinary team based on the DSM criteria 4th edition. The study population consisted of all children born in Israel 1002 to 2009, whose mother was an Israeli resident with a follow up period until end of December 2011[19].
In a population based study in Western Australia referring to children born from 1984 to 1999 ascertained through an establishment of a Central Diagnostic Panel with a cross-disciplinary reporting protocol aiming at a standardized diagnostic procedure using the versions of the Diagnostic and Statistical Manual of the American Psychiatric Association. Data sources were The Disability Services Commission of Western Australia database, the Western Australian Register of Autism Spectrum Disorders and a retrospective dataset based on an audit and individual case note review of all ASD cases born in Western Australia between 1984 and 1995 and diagnosed by 1999. The ethnic group allocation was based on the ethnic origin of the mother and not the father. The latter did not allow for a separation of children of mixed ethnic origin into a group with high genetic heterogeneity[20]
Comparison of prevalence of autism spectrum disorder in offspring between groups with high, moderate and low parental genetic heterogeneity
Table 1 contains the outcome data including numbers of children with ASD, the number in the reference populations, prevalence data and percentage of children with learning difficulties in the groups with ASD and in the foot note results of statistical comparisons of groups.
Comparison of autism prevalence in the subgroups of offspring of parents with high genetic heterogeneity and moderate genetic heterogeneity in the population of school children in the United Kingdom[14] showed a significantly higher odds of autistic spectrum disorder in children with high parental genetic heterogeneity of giving an odds ratio (OR): 1.18 (95% CI of 1.16 to 1.20). The odds for autism spectrum disorder in offspring of parents with moderate genetic heterogeneity was with 2.34 (95%CI 2.0 to 2.7) significantly higher than in offspring of parents with low genetic heterogeneity.
In the first study from the USA[15] odds of having autistic spectrum disorder in offspring of parents with high genetic heterogeneity was significantly higher than in offspring of parents with moderate genetic heterogeneity with an OR 1.15 (95%% CI 1.08-1.23) and higher in offspring of parents with moderate genetic heterogeneity compared to offspring of parents with low genetic heterogeneity with an OR 1.8 (9%%CI 1.62 to 2.00).
A subsequent population based survey in the USA[16] found a significantly higher odds of ASD in offspring of parents with high genetic heterogeneity compared to offspring of parents with moderate genetic heterogeneity with an odds ratio of OR: 1.44 (1.41-1.47). The survey deemed prevalence data for the group of Native American and Alaskan populations as not meeting the threshold for statistical precision for autism spectrum disorder prevalence (with the exception of the limited data from Arizona).
In the Swedish study the odds of ASD in offspring was significantly higher for high parental genetic heterogeneity versus moderate genetic heterogeneity with an odds ratio of 3.23 (1.75 to 5.94)[17].
In the population based study in Israel there was a significantly higher odds of ASD in the offspring of the general population of Israel with its high parental genetic heterogeneity compared to isolated groups of Israeli Arabs and ultraorthodox Jews with very low genetic heterogeneity with an OR of 3.81 (3.61 to 4.02)[19].
In a population based study from Australia the odds of autistic spectrum disorder was significantly higher in offspring of parents with moderate genetic heterogeneity versus low genetic heterogeneity with an odds ratio of 6.5 (3.5 to 12.3)[20].
Studies which did not allow statistical comparison of groups with different genetic heterogeneity
In one population based study from Alberta in Canada of 273343 live births analysed autism prevalence in different socioeconomic groups. The only ethnic group separately analysed was the aboriginal group (low genetic heterogeneity) which had a significantly reduced relative risk rate of ASD (RR = 0.58, 95% CI = 0.40-0.84) compared to a group not affected by low income, requiring welfare or where this was unknown[21].
Comparison of percentage of children with learning difficulties with the groups of children with autism spectrum disorder
Risk Ratio (RR) for learning difficulties in the population of autistic children was 0.79 (0.76 – 0.82) in high versus moderate parental genetic heterogeneity and 1.44 (1.14-1.83) in low versus moderate parental genetic heterogeneity in the study conducted in the United Kingdom. In the more recent study conducted in the USA it was 1.53 (95%CI 1.42 to 1.65) in offspring with autism for parents with high versus moderate parental genetic heterogeneity[16].
The report from Ireland demonstrated a significantly elevated risk ratio for learning difficulties in autistic offspring of parents of Sub Saharan African versus native Irish origin with a RR of 4.41 (2.27 to 8.56)[18].
In the population based study from Australia the percentage of children with learning difficulties amongst children with autism was not different between the two groups from this country with a risk ratio of 0.76 (0.55 to 1.04)[20].
Discussion
Key results: This study is the first to investigate the relationship of parental genetic heterogeneity to autism prevalence. It demonstrated that higher parental genetic heterogeneity is associated with an increase in ASD prevalence in the off-spring. A link of learning difficulties in offspring with ASD with parental genetic heterogeneity found support by data from the USA and Ireland but not from the United Kingdom or Australia.
Limitations: The study was limited to data from high income countries. These may have had better services for ethic subgroups like Caucasian parts of the population and detection of autism in population groups like Irish travellers, Roma, Israeli Arabs and Native Americans may have been affected by a reduced health care seeking behaviour. This does not explain the situation in the USA were ASD prevalence in the less affluent African American population was found to be significantly higher than in the Caucasian and Asian parts of the population. Health care has to be paid for in the USA and cost of health insurance may be prohibitively high for significant parts of the Afro-American population. The fact that despite the improvement in the economic situation of Native Canadian populations the autism prevalence has remained lower compared to other population groups in this country is against simple under-reporting due to lack of funded access to health care.
Interpretation
Previous research has focussed on the impact of inbreeding on prevalence of autistic spectrum disorders. One systematic review of prevalence of autism spectrum disorders and rate of consanginous marriages demonstrated that there was with an odds ratio of 1.78 (95% CI1.09, 2.47) a slightly increased risk of autism in consanguineous marriages[22].
If one compares countries with a known rate of consanginous marriages of less than 5%[23] and known autism prevalence like Western Europe, the USA, Australia and China it is clear that the autism prevalence in those countries ranges from 29 to 322 per 10000[6] while in countries with a higher percentage of consanginous marriages of above 10% including India and Saudi Arabia the prevalence of autism spectrum disorders ranges from 3.5 to 23 per 10000.
The results of the presented study can be interpreted as indicating a relationship of outbreeding to increase autism prevalence which particularly considering the comparison of highly endogamous groups like Irish travellers, or ultraorthodox Jews means an effect of inbreeding is outweighed by the effect of a lack of outbreeding. The effects of outbreeding have been investigated in other mammals[24,25] and can lead to reduced reproductive fitness (so called “outbreeding depression”). There is a significant reduction in number of offspring in spouses with ASD[26] .
This underscores the fact that the preservation of ASD genes in the human population must have been made possible by compensatory gene expression modifying genes which may now be uncoupled from the ASD genes they co-evolved with if there is high parental genetic heterogeneity.
Generalisability
It is unclear whether the effect of parental genetic heterogeneity is also observed without the influence of assortative mating e.g. in countries where arranged marriages predominate. Further research needs to explore this. Future studies also need to explore whether an increasing genetic heterogeneity of spouses within individuals of Caucasian and Ashkenazi ancestry in which has been detected in individuals included in the Framingham Heart Study (participants from Framingham, Massachusetts, USA)[27] is over time associated with increased ASD prevalence.
Appendix A
STROBE Statement—checklist of items that should be included in reports of observational studies
| Item No | Recommendation | ||
| Title and abstract | 1 | (a) Indicate the study’s design with a commonly used term in the title or the abstract | |
| (b) Provide in the abstract an informative and balanced summary of what was done and what was found | |||
| Introduction | |||
| Background/rationale | 2 | Explain the scientific background and rationale for the investigation being reported | |
| Objectives | 3 | State specific objectives, including any prespecified hypotheses | |
| Methods | |||
| Study design | 4 | Present key elements of study design early in the paper | |
| Setting | 5 | Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data collection | |
| Participants | 6 | (a) Cohort study—Give the eligibility criteria, and the sources and methods of selection of participants. Describe methods of follow-up Case-control study—Give the eligibility criteria, and the sources and methods of case ascertainment and control selection. Give the rationale for the choice of cases and controls Cross-sectional study—Give the eligibility criteria, and the sources and methods of selection of participants |
|
| (b) Cohort study—For matched studies, give matching criteria and number of exposed and unexposed Case-control study—For matched studies, give matching criteria and the number of controls per case | |||
| Variables | 7 | Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if applicable | |
| Data sources/ measurement | 8* | For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe comparability of assessment methods if there is more than one group | |
| Bias | 9 | Describe any efforts to address potential sources of bias | |
| Study size | 10 | Explain how the study size was arrived at | |
| Quantitative variables | 11 | Explain how quantitative variables were handled in the analyses. If applicable, describe which groupings were chosen and why | |
| Statistical methods | 12 | (a) Describe all statistical methods, including those used to control for confounding | |
| (b) Describe any methods used to examine subgroups and interactions | |||
| (c) Explain how missing data were addressed | |||
| (d) Cohort study—If applicable, explain how loss to follow-up was addressed Case-control study—If applicable, explain how matching of cases and controls was addressed Cross-sectional study—If applicable, describe analytical methods taking account of sampling strategy | |||
| (e) Describe any sensitivity analyses | |||
| Results | |||
| Participants | 13* | (a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, examined for eligibility, confirmed eligible, included in the study, completing follow-up, and analysed | |
| (b) Give reasons for non-participation at each stage | |||
| (c) Consider use of a flow diagram | |||
| Descriptive data | 14* | (a) Give characteristics of study participants (eg demographic, clinical, social) and information on exposures and potential confounders | |
| (b) Indicate number of participants with missing data for each variable of interest | |||
| (c) Cohort study—Summarise follow-up time (eg, average and total amount) | |||
| Outcome data | 15* | Cohort study—Report numbers of outcome events or summary measures over time | |
| Case-control study—Report numbers in each exposure category, or summary measures of exposure | |||
| Cross-sectional study—Report numbers of outcome events or summary measures | |||
| Main results | 16 | (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (eg, 95% confidence interval). Make clear which confounders were adjusted for and why they were included | |
| (b) Report category boundaries when continuous variables were categorized | |||
| (c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period | |||
| Other analyses | 17 | Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses | |
| Discussion | |||
| Key results | 18 | Summarise key results with reference to study objectives | |
| Limitations | 19 | Discuss limitations of the study, taking into account sources of potential bias or imprecision. Discuss both direction and magnitude of any potential bias | |
| Interpretation | 20 | Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from similar studies, and other relevant evidence | |
| Generalisability | 21 | Discuss the generalisability (external validity) of the study results | |
| Other information | |||
| Funding | 22 | Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on which the present article is based | |
| *Give information separately for cases and controls in case-control studies and, if applicable, for exposed and unexposed groups in cohort and cross-sectional studies. Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent reporting. The STROBE checklist is best used in conjunction with this article (freely available on the Web sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at http://www.annals.org/, and Epidemiology at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org. | |||
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Table 1.
Parental genetic heterogeneity and prevalence of autistic spectrum disorder and associated learning difficulties.
Table 1.
Parental genetic heterogeneity and prevalence of autistic spectrum disorder and associated learning difficulties.
| Country of origin | Prevalence (per 10000) of autism spectrum disorder with high parental genetic heterogeneity (n/N: number affected per total reference population) | Percentage of people with autistic spectrum disorder and learning difficulties and high parental genetic heterogeneity (n/N) | Prevalence of autism spectrum disorder (per 10000) with moderate parental genetic heterogeneity (n/N) | Percentage of people with autistic spectrum disorder and learning difficulties and moderate parental genetic heterogeneity (n/N) |
Prevalence of autism spectrum disorder (per 10000) with low parental genetic heterogeneity (n/N) | Percentage of people with autistic spectrum disorder and learning difficulties and low parental genetic heterogeneity (n/N) | Source of data |
| United Kingdom (n= 6,846,772) |
197.6a (15520/785352) |
14.11 (2190/15520) |
167.4b* (101062/6033808) |
17.78* (17969/101062) |
71.70 c(198/27612) |
25.7d 51/198 |
14. |
| United States of America (n=52408298) | 12.9a (12033/9256595) | N.D. | 9.0e* (38191/42434668) | N.D. | 5.0f(358/717035) | N.D. | 15 |
| United States of America (n=168853 | 280.5 a (1645/ 58622) |
48.7 (802/1645) |
243.1b,* (2680/110227) | 31.7* (852/2680) | N.D | N.D. | 16 |
| Sweden (Malmoe) (n=60756) | 107.9g(11/1019) | N.D. | 33.6h,i (201/59737) | N.D. | N.D. | N.D. | 17. |
| Ireland (n=73) | N.D. | 72.2j(13/18) | N.D. | N.D. | N.D. |
16.3ki,* (9/55) |
18 |
| Israel (n=2431649) | 56.3l (7469/1326303) | N.D. | N.D. | N.D. | 14.8m * (1640/1105346 | N.D. | 19 |
| Australia (n=374308 ) | N.D. | N.D. | 30.5n (1145/374308) | 60.8 (697/1145) | 4.6o,* (10/21506) |
80.0** (8/10) |
20 |
a Mixed (parents both from mixed or each parents from different ethnic back ground) or African ancestry, b Caucasian or Asian ancestry, cRoma or Irish traveller ancestry* p<0.0001 between prevalence groups and risk ratio for learning difficulties high versus moderate or low parental genetic heterogeneity,d p=0.0033 for moderate versus low genetic heterogeneity e Caucasian ancestry only as in this survey Asian populations were mixed with pacific islander of low genetic heterogeneity,f Native American and Alaskan populations. gSub-Saharan African, h Swedish, other Nordic countries, Western European, USA, Easter European, East Asian, i High versus moderate parental genetic heterogeneity p=0.0001, j African immigrants,k Native Irish population,.l General population in Israel, m Combined Israeli Arabs and ultraorthodox Jews. nCaucasian or Asian maternal origin,o Australian indigenous maternal origin, **p=0.183 for percentage with learning difficulties in the two groups.
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