Are ADHD and Autism Inherited? Research on Younger Siblings Indicates They Might Be

Study reveals that younger siblings of children with ADHD and autism are far more likely to develop one of those conditions.

Younger siblings of children with autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are at greater risk of developing those conditions.

A recent study published in JAMA Pediatrics found the odds of a sibling — born after a child diagnosed with ASD — being diagnosed with the same condition were 30 times higher than that of later-born siblings of nondiagnosed children.

The odds of a younger sibling of a child with ASD being diagnosed with ADHD were also 3.7 times higher when compared with siblings of nondiagnosed children.

For younger siblings of a child with ADHD, the odds of being diagnosed with the same condition were 13 times higher when compared with siblings of nondiagnosed children.

The odds of an ASD diagnosis for this group were 4.4 times higher.

“Compared to younger siblings of nondiagnosed children, we found that younger siblings of children with ASD were more likely to be diagnosed with ASD themselves, which is consistent with prior research,” Meghan Miller, PhD, author of the study and assistant professor in the Department of Psychiatry and Behavioral Sciences and at the UC Davis MIND Institute, told Healthline. “Also, as expected, we found that younger siblings of children with ADHD were more likely to be diagnosed with ADHD themselves. Perhaps most intriguingly, we found that younger siblings of children with ASD were at elevated risk for ADHD, and that younger siblings of children with ADHD were at elevated risk for ASD compared to younger siblings of nondiagnosed children.”

An inheritable condition

Miller and her colleagues examined medical records from 730 younger siblings of children living with ADHD and 158 later-born siblings of children living with ASD.

They also examined the records of 14,287 younger siblings of children without any known diagnosis.

ADHD and ASD are thought to share various genetic risk factors. Miller says previous research has shown the rates of a child having both ADHD and ASD are as high as 70 percent.

Miller’s research supports the idea that ADHD and ASD may share underlying causes and are highly inheritable.

“Prior research has strongly suggested genetic links between ASD and ADHD, based on the high comorbidity rates between these two conditions and evidence of shared heritability of the disorders and their symptoms. The research on specific overlapping genetic markers is mixed, and there is still much to be learned about the ways in which these two disorders are linked,” Miller said.

Children or adults living with ADHD may be hyperactive, compulsive, and have trouble paying attention.

ASD is characterized by problems with social, communication, or emotional skills. Children and adults living with ASD may behave, interact, and learn in ways that are notably different from others. They may struggle with a change in routine and may repeat certain behaviors.

Dr. Rolanda Gott, a developmental-behavioral pediatrician at the University of California Los Angeles Mattel Children’s Hospital, isn’t surprised by the findings of Miller’s study.

“ADHD and ASD may be different manifestation of the same overarching disorder that may manifest differently in multiple family members. Many genes affect multiple brain functions and it is more common that we see both ASD and ADHD than ASD alone,” Gott told Healthline.

“Many ADHD kids struggle with social skills deficits, inflexibility, sensory processing difficulties that are overlapping with ASD symptoms,” she added. “Both disorders are on a continuous spectrum and many times it is hard to draw the line between pure ADHD that also affects social communication and behavior versus ADHD plus ASD.”

Awareness and intervention

Although ASD and ADHD may appear differently, Miller’s research highlights the overlapping risk of the conditions for younger siblings as well as the importance of early monitoring.

“These findings suggest that younger siblings of children with ASD and ADHD should be monitored and screened for both disorders. We know that early intervention is important for children with ASD and those with ADHD,” Geraldine Dawson, PhD, director of the Duke Center for Autism and Brain Development told Healthline.

Scott Kollins, PhD, is a professor of psychiatry and behavioral sciences and director of the Duke ADHD Program. He says although much is known about the risk factors for both ADHD and ASD, there is still a lot to discover about the underlying causes of these conditions.

“There are many known genetic and nongenetic risk factors for both ASD and ADHD, including many common genetic variants, premature and/or low birthweight, and medical problems early in life. However, each of these known factors only contributes a very small amount of risk for having the disorders, so their actual clinical utility is limited,” he told Healthline.

There’s no medical test to diagnose ASD. A diagnosis is made when a doctor looks at the behavior and development of a child. Autism can be detected at 18 months, and by the time a child is 2, a diagnosis from an experienced doctor is very reliable.

However, many children are not diagnosed with ASD until they’re much older, and the delay in diagnosis means they may miss out on early help.

Similarly, there’s no single test for the diagnosis of ADHD. A diagnosis is made after an assessment of behavior against a set of criteria.

Parents shouldn’t worry too much

David Mandell, ScD, is the associate director of the Center for Autism Research at the Children’s Hospital of Philadelphia. He says monitoring of younger siblings is important and may also explain the level of diagnosis in this group.

“Younger children in families in which the older child has ADHD or ASD likely are subject to greater scrutiny. They may be more likely to get the diagnosis just because of increased surveillance, and the symptom bar may be lower than it is in other families,” Mandell told Healthline.

Although Miller’s research suggests a heightened risk for younger siblings, she says families with children who have the conditions who intend to have more children shouldn’t be worried.

“It’s important to note that most of the younger siblings of children with autism or ADHD did not receive diagnoses. So, although risk is increased among these younger siblings, most do not develop autism or ADHD. We suggest that more careful monitoring of younger siblings of diagnosed children may be useful so that, if signs or symptoms do begin to emerge, these younger siblings can be referred for early intervention as early as possible,” she said.