Siblings born in a family after other children with autism spectrum disorder (ASD) or attention-deficit/hyperactivity disorder (ADHD) were more likely to be diagnosed with the same disorder or the other disorder.

ADHD and ASD are common neurodevelopmental disorders that likely share some genetic factors and biological influences. Estimating recurrence risk in families is a way to measure shared genetic factors. Such risk estimates are often based on the total number of siblings in a family rather than being limited to later-born siblings (those born after children with ASD or ADHD) so that risk can be underestimated if families decide to stop having children after a child develops ASD or ADHD. This study focused on risk for later-born siblings.

 A total of 15,175 later-born siblings classified by familial risk based on an older child's diagnosis: ADHD risk (730), ASD risk (158) and no known risk (14,287); data were extracted from two large health care system in the United States.