In identical twins with ASD, symptom severity varies

A new study sheds light on the influence of nature and nurture on autism spectrum disorders (ASD) by showing that when identical twins have ASD, one twin frequently has more severe symptoms than the other. 

In the study, Lauren Castelbaum and colleagues analyzed data from three previous twin studies. In all, they looked at data from 366 identical twin pairs with and without ASD, looking at clinicians’ assessments or parents’ ratings on a standardized questionnaire, or both. 

The researchers found that if one twin had ASD, there was a 96% chance that the other did as well. However, the severity of symptoms varied widely in twin pairs. In fact, Castelbaum and colleagues estimated that genetic factors accounted for only 9% of the cause of trait variation between the twins. In contrast, the scores for traits were very similar for identical twins without ASD. 

The researchers say their study shows that “although ASD itself is highly heritable, variation in severity of symptomatology above the diagnostic threshold is substantially influenced, in contrast, by non-shared environmental factors which may identify novel targets of early ASD amelioration.”

Citations

“On the nature of monozygotic twin concordance and discordance for autistic trait severity: a quantitative analysis,” Lauren Castelbaum, Chad M. Sylvester, Yi Zhang, Qiongru Yu, and John N. Constantino, Behavior Genetics, December 18, 2019 (online). Address: John N. Constantino, [email protected]. 

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“Severity of autism symptoms varies greatly among identical twins,” news release, Eunice Kennedy Shriver National Institute of Child Health and Human Development, December 27, 2019.