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    Evidence that autistic traits show the same etiology in the general population and at the quantitative extremes (5%, 2.5%, and 1%)

    Robinson, E.B. and Koenen, K.C. and McCormick, M.C. and Munir, K. and Hallett, V. and Happé, F. and Plomin, R. and Ronald, Angelica (2011) Evidence that autistic traits show the same etiology in the general population and at the quantitative extremes (5%, 2.5%, and 1%). Archives of General Psychiatry 68 (11), pp. 1113-1121. ISSN 0003-990X.

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    - Context: Genetic factors play an important role in the etiology of both autism spectrum disorders and autistic traits. However, little is known about the etiologic consistency of autistic traits across levels of severity. - Objective: To compare the etiology of typical variation in autistic traits with extreme scoring groups (including top 1%) that mimicked the prevalence of diagnosed autism spectrum disorders in the largest twin study of autistic traits to date. - Design: Twin study using phenotypic analysis and genetic model-fitting in the total sample and extreme scoring groups (top 5%, 2.5%, and 1%). - Setting: A nationally representative twin sample from the general population of England. - Participants: The families of 5968 pairs aged 12 years old in the Twins’ Early Development Study. - Main Outcome Measure: Autistic traits as assessed by the Childhood Autism Spectrum Test. - Results: Moderate to high heritability was found for autistic traits in the general population (53% for females and 72% for males). High heritability was found in extreme-scoring groups. There were no differences in heritability among extreme groups or between the extreme groups and the general population. A continuous liability shift toward autistic trait affectedness was seen in the cotwins of individuals scoring in the top 1%, suggesting shared etiology between extreme scores and normal variation. - Conclusion: This evidence of similar etiology across normal variation and the extremes has implications for molecular genetic models of autism spectrum disorders and for conceptualizing autism spectrum disorders as the quantitative extreme of a neurodevelopmental continuum. Autism spectrum disorders (ASDs) are a set of phenotypically heterogeneous neurodevelopmental syndromes of primarily genetic etiology. Monozygotic (MZ) twins display from 60% to 90% concordance for ASD; the concordance in dizygotic (DZ) twins has been estimated from 0% to 30%.1- 7 This evidence suggests that ASD are one of the most highly heritable behavioral disorders. Modest to high heritability has been reported for autistic traits assessed quantitatively in the general population,8- 13 although assessments have varied in their estimates of genetic and environmental contributions.14- 21 Reported values of heritability vary from 36%18 to 87%.12,18 One hypothesis regarding the causes of ASD or extreme autistic traits is that the same variants that influence risk for extreme behavioral profiles also influence mild or subthreshold autism-like behavior.16,22- 23 Under this hypothesis, it is predicted that the etiologic structure of extreme autistic traits would be consistent across the range of impairment.24 Furthermore, if extreme traits are genetically linked to subthreshold variation, one would expect to see a shift toward affectedness in the continuous trait distribution of family members of extreme-scoring individuals, a shift that is dependent upon the family members' coefficient of genetic relatedness.25- 26 In other words, extreme scores should not only predispose family members to equally severe levels of impairment but also predict an increased liability toward mild or moderate autism-like behavior.17,26- 29 The etiology of extreme autistic traits (eg, >95th percentile) was examined in the present sample when the cohort was 8 years old.23 Those findings suggested that extreme autistic traits appeared to show similar etiology as diagnosed ASD. That study, however, was not large enough to examine an extreme group (top 1%) that shows a prevalence and average symptom burden similar to those of individuals with an ASD. In the present study, we use a sample that is 75% larger (n = 11 936) in order to examine—to our knowledge, for the first time—the etiologic consistency of autistic traits from the general population across a clinically comparable threshold. To test whether the etiology of extreme autistic traits was consistent across the range of impairment, heritability estimates were reported for the full sample and for individuals scoring in the top 5% (n = 615), 2.5% (n = 342), or 1% (n = 120) of the general population. Leveraging the size and clinical comparability of the top 1% group, this study also presents the first direct examination of whether a quantitative shift in sibling liability to less extreme impairment is associated with extremely severe affectation in a representative twin sample, a phenomenon that would be indicative of etiologic overlap between very extreme scores and regular variation in autism-like behavior.


    Item Type: Article
    School: Birkbeck Schools and Departments > School of Science > Psychological Sciences
    Research Centre: Brain and Cognitive Development, Centre for (CBCD)
    Depositing User: Administrator
    Date Deposited: 29 May 2013 16:42
    Last Modified: 02 Dec 2016 11:55


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