Autism and ADHD have lots of similarities, which often causes autism to be misdiagnosed as ADHD or sometimes vice versa. They can also co-occur in the same individual. The two disorders have long been believed to be linked, but this is the first time a study has shown that having a child with one or the other disorder increases the chances of your next child’s risk of either disorder, not necessarily the same one as their sibling has.
Researchers at the University of California, Davis, looked at medical records from two large healthcare systems and identified 15,175 children aged 5 or older who have at least one older sibling. Out of those children, 158 had an older sibling with autism and 730 had one with ADHD.
Those children whose older sibling had autism had a 30 times higher likelihood of developing autism as well, and those with a sibling with ADHD were at 13 times the normal likelihood of that diagnosis. But what’s even more interesting is that children whose older sibling had either of the conditions were also more likely to develop the other condition. Those with autistic older siblings were 3.7 times more likely to have ADHD, and those whose siblings had ADHD were four times more likely to be diagnosed with autism.
The results of the study, published in JAMA Pediatrics, seem to suggest not only that autism and ADHD are genetic disorders but also that they share genetic similarities. That is, the same gene codes could cause both autism and ADHD, either separately or together.
Similar previous studies have not taken into account that many parents of a child with ADHD or autism stop having children after that, an idea researchers refer to as “stoppage.” This leads to an artificially deflated rate of younger siblings being diagnosed with ADHD or autism, because some of the study’s participants did not have any younger siblings.
“By selecting families who have chosen to have another child after having a diagnosed child, we get a better sense of what really are the genetic contributions, and more reliable recurrence risk estimates,” says lead researcher Meghan Miller, assistant professor of psychiatry and behavioral sciences at the University of California, Davis.
The research team does acknowledge some limitations to their study, including the fact that most participants were white. The study also has not (yet) followed the children for an extended period of time to see how the features of ADHD and autism occur in the younger sibling.
Researchers are hopeful that their data could help further our understanding of the underlying cause of both autism and ADHD. The same research tactics may be able to be used in the future to examine autism alongside other comorbid disorders to determine whether there’s a genetic link there as well.
Elizabeth Nelson is a wordsmith, an alumna of Aquinas College in Grand Rapids, a four-leaf-clover finder, and a grammar connoisseur. She has lived in west Michigan since age four but loves to travel to new (and old) places. In her free time, she. . . wait, what’s free time?