Later-born siblings of children with autism spectrum disorder (ASD) or attention-deficit/hyperactivity disorder (ADHD) appear to be at elevated risk for the same disorder, but also of being diagnosed with the other disorder, a recent study found. Using data extracted from medical records of 2 large health care systems in the US, estimates of recurrence risk and cross-aggregation in later-born siblings of children with ADHD or ASD were compared with later-born siblings of children without these diagnoses. Researchers found:

• A total of 15,175 later-born siblings were classified by familial risk status based on the older child’s diagnostic status: ADHD risk (n=730; male [51.92%]), ASD risk (n=158; male [48.10%]), and no known risk (n= 14,287; male [50.73%]).
• Compared with later-born siblings of children without ADHD or ASD, later-born siblings of children with ASD were more likely to be diagnosed with ASD or ADHD in the absence of ASD.
• Compared with later-born siblings of children without a diagnosis, later-born siblings of children with ADHD were more likely to be diagnosed with ADHD or ASD in the absence of ADHD.