Despite the Same DNA, Severity of Autism Symptoms Varies Greatly Among Identical Twins

Identical Twin Girls

Findings from NIH-funded study could inform treatment strategies.

Identical twins with autism spectrum disorder (ASD) often experience large differences in symptom severity even though they share the same DNA, according to an analysis funded by the National Institutes of Health. The findings suggest that identifying the causes of this variability may inform the treatment of ASD-related symptoms. The study was conducted by John Constantino, M.D., of Washington University School of Medicine in St. Louis, and colleagues. Funding was provided by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). The study was published in Behavior Genetics on December 18, 2019.

ASD is a developmental disorder that affects how a person behaves, interacts with others and learns. Previous studies have found that when one identical twin has ASD, chances are extremely likely that the other twin has it, too.

The authors analyzed data from three previous studies comprising a total of 366 identical twin pairs with and without ASD. The severity of autism traits and symptoms in the twins was measured by a clinician’s assessment or by parents’ ratings on a standardized questionnaire. Some cases were diagnosed by both methods. The researchers determined a 96% chance that if one twin has ASD, the other has it, too. However, symptom scores varied greatly between twins diagnosed with ASD. The researchers estimated that genetic factors contributed to only 9% of the cause of trait variation among these twins. In contrast, among pairs of identical twins without ASD, the scores for traits were very similar.

The study authors do not know the reasons for differences in symptom severity, but they rule out genetic and most environmental causes because the twins share the same DNA and were raised in the same environment. Additional studies are needed to determine the cause.

Reference: “On the Nature of Monozygotic Twin Concordance and Discordance for Autistic Trait Severity: A Quantitative Analysis” by Lauren Castelbaum, Chad M. Sylvester, Yi Zhang, Qiongru Yu and John N. Constantino, 18 December 2019, Behavior Genetics.
DOI: 10.1007/s10519-019-09987-2

4 Comments on "Despite the Same DNA, Severity of Autism Symptoms Varies Greatly Among Identical Twins"

  1. Suggest it could be connected to interaction between the identical twins. I was once in a relationship with an identical twin who had an American accent (I am UK). After a year I met her twin. The problem I had initially was telling the twins apart when one was talking to her mother in the adjacent room with the same accent. It soon clicked – my partner’s sister had a whiny tone to her voice. The twins themselves put their personality differences down to womb position, an explanation of which I was sceptical. I thought it was due to early childhood interaction between the pair

  2. Autism in identical twins may not be caused by DNA, but might be caused by some chemical they both were exposed to in the womb.

  3. I am a twin. My own observations of twins over the years nearly always confirms that one twin is more social and one more introverted. This natural disparity might be playing out in their symptoms.

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