Key Point: Study concludes environment during gestation or shortly after is the biggest factor.

Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism

Joachim Hallmayer, MD; Sue Cleveland, BS; Andrea Torres, MA; Jennifer Phillips, PhD; Brianne Cohen, BA; Tiffany Torigoe, BA; Janet Miller, PhD; Angie Fedele, BA; Jack Collins, MBA; Karen Smith, BS; Linda Lotspeich, MD; Lisa A. Croen, PhD; Sally Ozonoff, PhD; Clara Lajonchere, PhD; Judith K. Grether, PhD; Neil Risch, PhD

Arch Gen Psychiatry. Published online July 4, 2011.

(See the excerpted AP article posted below this abstract for a simpler explanation.)

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).
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.

An Associated Press article on the research simplifies the results for the general public:

“The new study included 192 sets of twins where at least one of the twins was affected with autism. Some of the twins were identical and some were non-identical, or fraternal, twins.

The researchers used DNA testing to determine which twins were identical and which were fraternal. That was important because identical twins come from one fertilized egg and have identical genetic makeups. Fraternal twins, from two fertilized eggs, share no more genetic material than any other siblings.

The new study found, as expected, high rates of shared autism disorders for identical twins: 77 percent of male twin pairs and 50 percent for female pairs had autism in both twins.

Surprisingly, it also found fairly high rates of fraternal twins both having autism spectrum disorders: 31 percent rate for male fraternal twins and 36 percent for female fraternal twins.

Other studies have found 10 to 20 percent of younger siblings of children with autism are likely to be diagnosed themselves with the disorder.

Fraternal twins share the same womb, even though they don’t share identical genes. That could be important, said Dr. John Constantino of Washington University School of Medicine in St. Louis, who wasn’t involved in the new research.

‘Finding so many fraternal twin pairs in whom both twins have autism spectrum disorders is a key finding that puts a spotlight on pregnancy as a time when environmental factors might exert their effects,’ Constantino said.”

See the entire AP article at