Relative contributions of genetic and environmental factors to autism and disease traits have remained stable over decades, large twin study finds 1.
Among tens of thousands of Swedish twins born in 26 years, genetic factors have always had a greater impact on the development of autism and autism traits than environmental factors. The study suggests that genetics represent about 93% of the chance that a person has autism and 61 to 73% of the chance that they have autistic traits.
The numbers correspond to previous work showing that genetics exert a disproportionate influence on the odds of autism. The results also indicate that environmental factors are unlikely to explain the increased prevalence of autism. Otherwise, their contribution to autism in twins would also have increased over time.
“I think the relative consistency of the genetic and environmental factors underlying autism and autism traits is the most important aspect of this work,” says Mark Taylor, Senior Research Specialist at the Karolinska Institutet in Stockholm, Sweden, who led the study. “Prior to our study, there had been no twin studies examining whether the genetic and environmental factors underlying autism had changed over time.”
The researchers analyzed data from two sources: 22,678 pairs of twins in the Swedish twins register, who were born from 1982 to 2008; and 15,280 pairs of twins from Study of twins of children and adolescents in Sweden, born from 1992 to 2008.
The team divided these groups into cohorts based on the children’s date of birth. They created five birth cohorts from the registration group and four from the study group. Cohorts include both fraternal and identical twins. Identical twins share almost all of their genetic code, while fraternal twins share an average of 50 percent. In the registry group, about 24% of twin pairs are identical; in the study group, almost 30 percent are identical.
Among these pairs of twins, they identified children with autism and confirmed the diagnoses, either using medical records and case notes or through telephone interviews with parents or caregivers. Also through telephone interviews, they identified twins in the study group who exhibit autistic traits but do not meet the criteria for a diagnosis.
To estimate how much genetics or environment contributed to autism in the two groups, they compared the differences between identical and fraternal pairs of twins, specifically looking at how often one or both twins were affected. . Then they looked to see if these proportions varied from one birth cohort to another.
In all birth cohorts, there are more pairs of identical twins than fraternal pairs in which both twins are autistic or have autistic traits. The book appeared on May 6 in JAMA Psychiatry.
The model suggests that genetic factors have more influence on autism than environmental factors. If the reverse were true, there would be more similarities in behavior between fraternal twins.
Some experts wonder if twin studies – even large-scale ones like this one – can offer definitive answers.
The origins of autism may involve an interaction between genetics and the environment, which the twin studies do not capture, says Brian Lee, associate professor of epidemiology and biostatistics at Drexel University in Philadelphia, Pa., Who did not participate in the study.
“Asking if genetics or the environment is more important is the wrong question,” says Lee. “For me, it’s a bit like asking if it’s more important to put on your left shoe or your right shoe when going out in public.
The new study is well designed and conducted but, given the limitations of twin studies, may not add much to previous work to clarify the relationship between environment, genetics and autism, says Joachim hallmayer, professor of psychiatry at Stanford University in California, who was not involved in the study.
“It’s probably as good as these studies in our field can be,” he says.
Taylor disagrees and says the findings could shape the discussion about increasing autism prevalence figures.
“[This study] points out that despite the increased rate of autism diagnoses, it really doesn’t seem like much has changed in the underlying etiology, ”he says.
Next, the team plans to focus on specific environmental factors they previously associated with autism to see if those contributions have changed over time. They also plan to look at other neurodevelopmental disorders that have become more or less prevalent.