Heritability of autism spectrum disorders: a meta‐analysis of twin studies

<jats:sec> <jats:title>Background</jats:title> <jats:p> The etiology of Autism Spectrum Disorder ( <jats:styled-content style="fixed-case">ASD</jats:styled-content> ) has been recently debated due to emerging findings on the importance of shared environmental influences. However, two recent twin studies do not support this and instead re‐affirm strong genetic effects on the liability to <jats:styled-content style="fixed-case">ASD</jats:styled-content> , a finding consistent with previous reports. This study conducts a systematic review and meta‐analysis of all twin studies of <jats:styled-content style="fixed-case">ASD</jats:styled-content> published to date and explores the etiology along the continuum of a quantitative measure of <jats:styled-content style="fixed-case">ASD</jats:styled-content> . </jats:p> </jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p> A PubMed Central, Science Direct, Google Scholar, Web of Knowledge structured search conducted online, to identify all twin studies on <jats:styled-content style="fixed-case">ASD</jats:styled-content> published to date. Thirteen primary twin studies were identified, seven were included in the meta‐analysis by meeting Systematic Recruitment criterion; correction for selection and ascertainment strategies, and applied prevalences were assessed for these studies. In addition, a quantile <jats:styled-content style="fixed-case">DF</jats:styled-content> extremes analysis was carried out on Childhood Autism Spectrum Test scores measured in a population sample of 6,413 twin pairs including affected twins. </jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p> The meta‐analysis correlations for monozygotic twins ( <jats:styled-content style="fixed-case">MZ</jats:styled-content> ) were almost perfect at .98 (95% Confidence Interval, .96–.99). The dizygotic ( <jats:styled-content style="fixed-case">DZ</jats:styled-content> ) correlation, however, was .53 (95% <jats:styled-content style="fixed-case">CI</jats:styled-content> .44–.60) when <jats:styled-content style="fixed-case">ASD</jats:styled-content> prevalence rate was set at 5% (in line with the Broad Phenotype of <jats:styled-content style="fixed-case">ASD</jats:styled-content> ) and increased to .67 (95% <jats:styled-content style="fixed-case">CI</jats:styled-content> .61–.72) when applying a prevalence rate of 1%. The meta‐analytic heritability estimates were substantial: 64–91%. Shared environmental effects became significant as the prevalence rate decreased from 5–1%: 07–35%. The <jats:styled-content style="fixed-case">DF</jats:styled-content> analyses show that for the most part, there is no departure from linearity in heritability. </jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p> We demonstrate that: (a) <jats:styled-content style="fixed-case">ASD</jats:styled-content> is due to strong genetic effects; (b) shared environmental effects become significant as a function of lower prevalence rate; (c) previously reported significant shared environmental influences are likely a statistical artefact of overinclusion of concordant <jats:styled-content style="fixed-case">DZ</jats:styled-content> twins. </jats:p> </jats:sec>