Children who have an older sibling with attention deficit hyperactivity disorder (ADHD) are at an increased risk of having autism, a new study suggests. 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.
Miller and her colleagues looked at medical records from two large healthcare systems in the United States. They identified 15,175 children aged 5 or older who have at least one older sibling; of these children, 158 have an older sibling with autism and 730 have one with ADHD.
Children who have an autistic older sibling have 30 times the odds of having autism compared with children who have a typical older sibling; children who have an older sibling with ADHD have 13 times the odds of having ADHD themselves.
“Both these outcomes confirm the familial factor in these neurodevelopmental disorders,” says Tinca Polderman, assistant professor of complex trait development at Vrije Universiteit Amsterdam in the Netherlands, who was not involved in the work. But the scale of the autism effect is “astonishing,” she says.
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