WEDNESDAY, May 27, 2020 (HealthDay News) -- A child with an uncle or aunt with autism appears to have a more than doubled risk of being diagnosed with an autism spectrum disorder themselves, a new U.S. government-funded study reports.
Roughly 3% to 5% of children with an aunt or uncle with autism can also be expected to have some form of autism, compared with just 1.5% of children overall, according to the study funded by the U.S. National Institutes of Health.
However, researchers portray this as reassuring news for a person with a brother or sister with autism who is thinking about starting a family.
A couple who've had one child with autism have a 20% to 50% chance that later siblings also will be diagnosed with an autism spectrum disorder (ASD), said study co-author Dr. John Constantino, director of child and adolescent psychiatry at the Washington University School of Medicine in St. Louis.
"On average, these results are a potential source of reassurance to siblings of individuals with autism, in terms of having their own children," Constantino said. "It shows the risk is elevated, but not dramatically."
Autism is a complex neurodevelopmental disorder that begins in early childhood and affects communication, social skills and learning.
The study results also cast doubt on a theory of autism that holds that girls have built-in resistance to ASD-related genes, potentially explaining why three times as many boys are diagnosed with autism as girls, researchers added.
The study found no statistically significant difference between the genetic risk from mothers with a sibling with autism compared with the genetic risk from fathers with a sibling with autism.
According to senior researcher Sven Sandin, a statistician and epidemiologist with the Karolinska Institute in Stockholm, Sweden, "If the female protective effect hypothesis is true, then this ratio should be larger. We should be seeing this in the population."
A huge share of a person's risk for autism -- as much as 85% -- comes from their genetics, Constantino said.
If a female protective effect exists, many women who carry such risk factors would remain unaffected but then pass these ASD-related genes to their sons, the researchers said.
In that case, the children of mothers with an autism-diagnosed sibling could be expected to have as much as a 30% higher risk of ASD, Constantino said.
"Since the prevailing theory about sex differences would predict a much higher rate of autism among the offspring of a sister, the sister should be relatively reassured that their rate is not that high in relation to the general population," Constantino said of the study's results.
To test this theory, the researchers tracked health data from nearly 850,000 children born in Sweden between 2003 and 2012, and their families.
Roughly 13,000 of those children were diagnosed with an autism spectrum disorder, about 1.5% of all the kids born during that period.
The investigators found no significant difference between men and women with a sibling with autism. They also found that, while increased, the genetic risk of ASD that comes with having an aunt or uncle with autism did not exceed what one would expect based on previous studies related to the role of genetics in autism.
The researchers' conclusions related to a lack of a female protective effect are "plausible," said Kristen Lyall, an assistant professor with the Drexel University Autism Institute in Philadelphia.
If that's the case, researchers will have to look for other explanations for the gender difference between boys and girls, Lyall said.
Boys might be more susceptible to autism, or there might be underdiagnosis of autism among girls, Lyall said.
"Each of these explanations may be in play, to some extent," Lyall said. "There may be some combination of factors going on. I think it would be really interesting in future studies to address this topic."
People with siblings or parents with autism shouldn't let the results of this study influence their plans to have children, Lyall added.
"It's premature to apply the results of a single study to something as sensitive and complex as family planning," Lyall said.
The new study was published online recently in the journal Biological Psychiatry.