In this study using 2 sets of population-based medical records, researchers found that children whose older brother or sister had autism were 30 times more likely to receive the same diagnosis and 3.7 times more likely to be diagnosed with ADHD than children whose older sibling did not have either disorder. Led by Meghan Miller, PhD, of the University of California, Davis MIND Institute, the team discovered that similarly, children with an older sibling who had ADHD were 13 times more likely to receive an ADHD diagnosis and 4.4 times more likely to be diagnosed with autism than children in the control group, they wrote in JAMA Pediatrics.