Identical twins with autism spectrum disorder (ASD) may experience large differences in symptom severity even though they share the same genes, according to a study.

The findings, published in the journal Behavior Genetics, suggests that identifying the causes of this variability in symptoms may inform the treatment of ASD.

According to the researchers, including John Constantino of Washington University in the US, ASD is a developmental disorder affecting how a person behaves, interacts with others, and learns.

Earlier studies had found that when one identical twin has ASD, there are high chances that the other twin has it, too.

In the current study, the scientists assessed data from three previous studies comprising a total of 366 identical twin pairs with, and without ASD.

They measured the severity of autism traits and symptoms in the twins using a clinician's assessment or by parents' ratings on a standardised questionnaire with some cases diagnosed by both methods.

Based on their assessment, the researchers estimated a 96 per cent chance that if one twin had ASD, the other may have it, too.

However, they said the symptom scores varied greatly between twins diagnosed with ASD, adding that genetic factors contributed to only 9 per cent of the cause of trait variation among these twins.

On the contrary, among pairs of identical twins without ASD, the scores for traits were very similar.

While the authors could not find the reasons for differences in symptom severity, they could rule out genetic and most environmental factors since the studied twins shared the same DNA, and were also raised in the same environment.

The scientists concluded that additional studies are needed to determine the cause for differences in symptom severity among the twins with ASD.