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Study: Heredity and Environment Play Equal Role in Autism

Date Published: 
May 6, 2014

A large study in Sweden found that heredity accounted for half of the risk of autism, with unspecified environmental factors accounting for the rest of the risk in a population.

Researchers examined the records of 2 million Swedish children to determine the individual risk of autism in siblings, half-siblings and cousins in families affected by the disorder and those who are not. A sibling of a child with autism spectrum disorder (ASD) has about 10 times the risk of someone whose sibling does not have autism, according to research published in the Journal of the American Medical Association this month.1

By age 20, someone whose "full sibling" has ASD has a 13 percent probability of having ASD himself, compared to 1 percent for someone whose brother or sister does not have ASD. The probability of having autism for an identical twin of someone with autism is almost 60 percent. The probabiliy is about 13 percent for a fraternal (nonidentical) twin, from 7 to 9 percent for half-siblings, and 3 percent for cousins.

Other researchers have suggested that girls with autism come from families with a greater genetic risk for it, but this study did not find any family risk differences between girls and boys. The study team, led by Sven Sandin, examined the records of 2,049,973 children born in Sweden from 1982 through 2006.