Children who have an older sibling with attention deficit hyperactivity disorder (ADHD) are at an increased risk of having autism, a new study suggests¹. The reverse is also true.

Autism and ADHD share some features and often co-occur. Other studies have documented the increased risk in siblings for each of the conditions individually. But this is the first study to look at both conditions at once by homing in on the risk in younger siblings.

The results bolster the idea that there is a significant genetic overlap between the conditions.

The study’s findings are reliable because of its focus on later-born siblings, says lead researcher Meghan Miller, assistant professor of psychiatry and behavioral sciences at the University of California, Davis. Other studies may show an artificially low recurrence for these conditions because parents who have one diagnosed child sometimes choose not to have any more children — a phenomenon known as ‘stoppage.’

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