The findings, published in the journal Behaviour Genetics, suggest that identifying the causes of this variability may inform the treatment of ASD-related symptoms.
Autism spectrum disorder 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 autism spectrum disorder, spectrum disorder chances are extremely likely that the other twin has it, too.
For the findings, the researchers analysed 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 standardised questionnaire.
Some cases were diagnosed by both methods. The researchers determined a 96 per cent 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 nine per cent 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.
According to the study, the 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, the researchers said.
( With inputs from IANS )