Around one in every 100 children worldwide has autism spectrum disorder; in the U.S., around one in 88. For years, researchers have suspected that a complex and varying interplay between genetic factors and the environment causes the disorder, and now the results of a long-term study, published in JAMA, indicate that environment and genetics contribute about equally to a child's risk of developing ASD, the Scientist reports.
The study followed more than 2 million newborns in Sweden from 1982 until 2006. Of those kids, 14,516 were diagnosed with ASD. The researchers found that children who had a sibling or relative diagnosed with ASD had an elevated chance of inheriting that condition themselves. The researchers were able to quantify that risk, however, calculating that a child who has a sibling with autism has a ten-times greater risk of developing ASD, while a child whose cousin has autism has a two-times greater risk, the Scientist writes.
But that risk was lower than previous studies have found. Reuters reports:
The study involved two separate measures of autism risk – heritability, which is the proportion of risk in the population that can be attributed to genetic factors, and relative recurrent risk which measures individual risk for people who have a relative with autism.
Most previous studies have suggested heritability of autism may be as high as 80 to 90 percent. But this new study, the largest and most comprehensive to date, found genetics factors only explained around half of the cause of the disorder.
As the Telegraph points out, many of the children had no family history of autism at all, and the researchers told the paper that they were surprised to find that the environment plays as strong a role as genetic factors. “Recent research efforts have tended to focus on genes, but it’s now clear that we need much more research to focus on identifying what these environmental factors are,” they said.
In the meantime, parents who have already had a child with autism or whose brother or sister has had a child with autism can now make more informed family planning decisions by taking those risk factors into account. So while the exact environmental factors contributing to autism still largely remain elusive, when it comes to heritability, as the researchers told Gen News, “we can now provide accurate information about autism risk which can comfort and guide parents and clinicians in their decision.”