Later sibling recurrence of ASD and ADHD: clinical and mechanistic insights

In this issue of JAMA Pediatrics, Miller and colleagues1 report on the within- and cross-condition recurrence of attention-deficit/hyperactivity disorder (ADHD) or autism spectrum disorder (ASD) in later-born siblings of children (probands) with those conditions. The relatively high heritability of both conditions is well established from twin studies assessing both trait variance2 and diagnostic recurrence.3 Two recent large-scale Scandinavian studies extended this work to show that the 2 conditions also occur together in families.4,5 One population-based study of children with ASD (N = 3578) estimated the combined (older and younger) sibling recurrence of ASD in siblings to be 10.5%, but also reported that 5.3% had siblings with a diagnosis of ADHD, giving relative risks of 11.8 and 3.7 compared with matched controls without ASD.