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Original Article
Nov 2011

Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism

Author Affiliations

Author Affiliations: Department of Psychiatry, Stanford University School of Medicine, Stanford (Drs Hallmayer, Lotspeich, and Phillips and Mss Cleveland and Torres), Autism Genetic Resource Exchange, Autism Speaks, Los Angeles (Mss Cohen, Torigoe, and Fedele and Drs Miller and Lajonchere), Environmental Health Investigations Branch, California Department of Public Health, Richmond (Mr Collins, Ms Smith, and Dr Grether), Division of Research, Kaiser Permanente Northern California, Oakland (Drs Croen and Risch), University of California, Davis, MIND Institute, Sacramento (Dr Ozonoff), and Institute for Human Genetics and Department of Epidemiology and Biostatistics, University of California, San Francisco (Dr Risch).

Arch Gen Psychiatry. 2011;68(11):1095-1102. doi:10.1001/archgenpsychiatry.2011.76

Context Autism is considered the most heritable of neurodevelopmental disorders, mainly because of the large difference in concordance rates between monozygotic and dizygotic twins.

Objective To provide rigorous quantitative estimates of genetic heritability of autism and the effects of shared environment.

Design, Setting, and Participants Twin pairs with at least 1 twin with an autism spectrum disorder (ASD) born between 1987 and 2004 were identified through the California Department of Developmental Services.

Main Outcome Measures Structured diagnostic assessments (Autism Diagnostic Interview–Revised and Autism Diagnostic Observation Schedule) were completed on 192 twin pairs. Concordance rates were calculated and parametric models were fitted for 2 definitions, 1 narrow (strict autism) and 1 broad (ASD).

Results For strict autism, probandwise concordance for male twins was 0.58 for 40 monozygotic pairs (95% confidence interval [CI], 0.42-0.74) and 0.21 for 31 dizygotic pairs (95% CI, 0.09-0.43); for female twins, the concordance was 0.60 for 7 monozygotic pairs (95% CI, 0.28-0.90) and 0.27 for 10 dizygotic pairs (95% CI, 0.09-0.69). For ASD, the probandwise concordance for male twins was 0.77 for 45 monozygotic pairs (95% CI, 0.65-0.86) and 0.31 for 45 dizygotic pairs (95% CI, 0.16-0.46); for female twins, the concordance was 0.50 for 9 monozygotic pairs (95% CI, 0.16-0.84) and 0.36 for 13 dizygotic pairs (95% CI, 0.11-0.60). A large proportion of the variance in liability can be explained by shared environmental factors (55%; 95% CI, 9%-81% for autism and 58%; 95% CI, 30%-80% for ASD) in addition to moderate genetic heritability (37%; 95% CI, 8%-84% for autism and 38%; 95% CI, 14%-67% for ASD).

Conclusion Susceptibility to ASD has moderate genetic heritability and a substantial shared twin environmental component.