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Heather Ingraham of Barrie, Ontario, has three boys -  Dakota, age 14, Jacob, age 11, and Deacon, age eight. All three siblings have been diagnosed with Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD).

“Dakota was very behind in everything,” Ingraham told us. “He didn't crawl until he was one, didn’t walk until he was two and a half, and didn’t speak until he was four. He didn't interact at all, it was almost like he was staring at a wall. He hated being held or cuddled. He would fixate on light switches, turning them on and off and would fixate on spinning wheels on strollers, and on bikes. He was always rocking back and forth and climbing anything he could.  His main issue now is that he flips out over not getting his own way.”

Ingraham says though her son Jacob “started off great”, he began showing signs of ASD and ADHD by age two.

“He was hitting milestones early,” Ingraham told us. “He was walking at 11 months and talking in full sentences by 11-12 months. But by age two, he just quit walking and reverted back to army crawling. He quit talking and would take your hand and lead you to a toy if he wanted it. He would squeal or grunt to communicate.”

Like his older brother, Jacob was also fixated on items with wheels. He was diagnosed with ASD and ADHD around age four.

“Like Jacob, Deacon started off ahead for his age,” Ingraham told us. “He was walking and talking by 11 months and showed no signs of ASD at all except by age four when he became very aggressive and defiant. He began attacking his baby sister, his dad and I, his teachers and his classmates. He was diagnosed with ADHD and ASD at age seven.”

According to a new study published in JAMA Pediatrics, compared to later-born siblings of non-diagnosed children, later-born siblings of children with autism were more likely to be diagnosed with autism or with ADHD. Likewise, compared to later-born siblings of non-diagnosed children, later-born siblings of children with ADHD were more likely to be diagnosed with ADHD or with autism.

“We've known for quite some time that risk for autism is elevated among younger siblings of children with autism, and that autism and ADHD frequently co-occur,” study author, Meghan Miller told us. Miller is an Assistant Professor in the Department of Psychiatry and Behavioral Sciences
at the UC Davis MIND Institute in California. “Prior research has also found that mothers with ADHD are more likely to have a child with ADHD than mothers without ADHD, but also more likely to have a child with autism. So we've known of a relationship between autism and ADHD for some time.”

Miller says despite the available research, there wasn’t “good” data on recurrence risk for ADHD among younger siblings of children with ADHD, despite knowing that ADHD is also highly heritable, nor was it known much about how these conditions run across families.

The current study, titled, Sibling Recurrence Risk and Cross-aggregation of Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder, used medical data from the United States covering a time period from 1995 to 2018.

“In order to obtain unbiased recurrence risk estimates, it was important to address these gaps in knowledge in a sample that consisted of families who had at least one subsequent child after the diagnosis of an older child,” Miller told us. “These findings provide further support for shared familial mechanisms underlying these two disorders, which appear to be largely genetic in nature.” 

Miller says the findings suggest that younger siblings of children with either diagnosis should be monitored for both conditions throughout early childhood.

“However, it's important to note that most of the younger siblings of children with autism or ADHD did not receive diagnoses,” Miller told us. “So, although risk is increased among these younger siblings, most do not develop autism or ADHD.”