Background: What is the autism spectrum disorder (ASD) risk for a child who has an older sibling with ASD? Most researchers agree that a mix of genetics and environment affects risk, but they disagree on how much each factor plays a part. This makes risk prediction tricky. Previous reports indicate a wide range in ASD recurrence—when a second child has ASD.

What’s New: In a population-based study, researchers observed ASD recurrence rates in full- and half-siblings. The research team used records for all children, about 1.5 million, born in Denmark between January 1, 1980 and December 31, 2004.

The researchers report an overall recurrence risk of 7 percent. This is significantly higher than the population ASD risk of about 1.2 percent. For half-siblings sharing a father, recurrence rates were similar to the general population risk. In contrast, 2.4 percent of maternal half-siblings had ASD.

Why It’s Important: This study indicates a somewhat lower rate of recurrence than previously reported, a valuable finding for parents. A greater recurrence rate in maternal half-siblings suggests a key role for fetal environment in ASD risk. Also, the smaller recurrence risk in half-siblings, who share fewer genes, confirms a significant genetic contribution.

Risk Factors

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