Sibling Study Strengthens Genetic Link Between Autism and Attention Deficit | Spectrum
Children who have an older sibling with attention deficit hyperactivity disorder (ADHD) are at increased risk of autism, new study finds1. The reverse is also true.
Autism and ADHD share certain characteristics and often coexist. 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 together by focusing on the risk in younger siblings.
The results reinforce the idea that there is significant genetic overlap between the conditions.
The study results are reliable because they focus on siblings born later, says lead researcher Meghan Miller, an assistant professor of psychiatry and behavioral sciences at the University of California, Davis. Other studies may show artificially low recurrence for these conditions because parents who have a diagnosed child sometimes choose not to have any more children – a phenomenon known as ‘dropout’.
“By selecting families who chose to have another child after having a diagnosed child, we get a better idea of what the genetic contributions really are and more reliable estimates of risk for recurrence,” says Miller.
The study design also provides a powerful way to analyze other conditions that occur alongside autism, says Benjamin Yerys, assistant professor of psychology in psychiatry at Children’s Hospital of Philadelphia, who has no participated in the study. “[It’s] a new way to fetch this information that others would be smart to reproduce.
Miller and his colleagues reviewed medical records from two major health 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 brother with autism and 730 have one with ADHD.
Children who have an autistic older brother are 30 times more likely to have autism than children who have a typical older brother; children who have an older brother with ADHD are 13 times more likely to have ADHD themselves.
“These two results 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 magnitude of the autism effect is “astonishing,” she says. Previous studies pegged this increase at 14 to 20 times.
Having an older sibling with either condition also increases a child’s chance of having the other condition: children with older siblings with autism have 3.7 times more likely to have ADHD, and those with an older sibling with ADHD are four times more likely to have autism. . The findings were published in December in JAMA Pediatrics.
The children in the study are predominantly white. Non-white children are more likely to be diagnosed with ADHD than with autism, so it would be worth repeating the study in a more diverse group, Yerys says.
Miller and his colleagues follow children who have an older sibling with either condition to track the onset of the features early in life.
Boys are more likely than girls to be diagnosed with either disease, and children with older mothers are more likely than those with younger mothers to be diagnosed with autism. These results are all in agreement with those of previous studies.
However, the new study found that having an older mother reduces the chances of having ADHD. The reason for this finding is unclear.
Overall, the findings suggest that physicians should look beyond the older sibling’s condition when evaluating the younger sibling, Yerys says. As a clinician, he says, “it’s about keeping your eyes open and not making assumptions.”
Still, Polderman cautions, the absolute risk of having a second child with either condition is low. The new study could inflate the risk because parents with a diagnosed child may be more likely to scrutinize milestones than other parents.