Do the seeds of neurodiversity come from the family tree?
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
It’s a question we’ve been asking in one form or another since the early 1900s, when psychiatrists first began writing about the collection of symptoms we now refer to as autism spectrum disorder (ASD).
Over a century later, our understanding of autism has grown by leaps and bounds. But our knowledge remains incomplete. We can definitively say that yes, autism does run in families, but the “how” and “why” are complicated, to say the least. In fact, as many as 1,000 different genetic changes may affect how brain cells communicate in people with autism.
We talked to developmental pediatrician Carrie Cuffman, MD, to learn more.
Genes vs. inheritable traits vs. heritable traits
Before jumping into a topic this complex, let’s make sure we’re on the same page. You’ve probably heard genetics, heritability and inheritability discussed like they’re interchangeable, but that’s not actually right.
- The genes we get directly from our biological parents and grandparents — like our eye or hair color — are inherited.
- Heritable traits are not genetic. They’re things like habits, skills or behaviors that we often share with our relatives. They’re there, but they aren’t encoded in our DNA.
When it comes to the genetics of ASD, we’re mostly talking about inheritable traits — a single gene or collection of genes that impact a child’s development. Of course, environmental factors can also play a role in how those traits get expressed, but that’s a conversation for another day.
To make things even more complicated, there are many causes of autism that are genetic but not inherited — meaning that there’s a change in a child’s genes that’s new and not seen in their parents. These genetic causes of autism are more likely to impact a single gene or single region of a chromosome. More on this in a bit.
Autism in siblings
When it comes to siblings, the research is clear: When one child is diagnosed with ASD, the next child to come along has about a 20% greater chance of developing ASD than a child with neurotypical older siblings. And when the first two children in a family receive that diagnosis, the chances go up to 32%.
There’s some evidence to suggest that the sex assigned at birth changes the probability, too, but more research needs to be done to confirm it.
Statistics are great, but they don’t give us much insight into the lived reality of raising siblings who have ASD. That’s because autism symptoms and severity vary from child to child.
There are many kinds of autism
Neurodiversity is just that: Diverse. It’s important to keep that in mind when thinking about autism in families.
Why? When we talk about autism, we’re talking about a diagnostic umbrella that covers many different kinds of developmental disorders. The role genetics play in autism varies based on the condition in question. There isn’t a single “autism gene” healthcare providers can test for. In fact, it’s not always possible to identify a genetic cause for autism.
It’s also important to note that genetics can’t necessarily determine the severity of symptoms. In other words: Two siblings with the same autism diagnosis will likely experience the condition differently.
Single-gene disorders vs. multi-gene disorders
Perhaps the best-known single-gene disorder that causes ASD is fragile X syndrome. Other single-gene disorders include Rett syndrome and tuberous sclerosis.
“Some disorders related to autism are very clearly hereditary and would be passed down based on the genetics of the individual,” Dr. Cuffman explains.
While there are no hard-and-fast rules, it’s often the case that children with single-gene disorders have severe autism symptoms, as well as other health issues or impairments. Put simply: It’s less likely that a child whose autism is the result of a single-gene disorder will have to wait years for a diagnosis because the symptoms are more severe and the cause can be easier to identify.
As you may have already guessed, the majority of children with ASD don’t have a single-gene disorder. At the population level, it’s more common for multiple different genes working together — possibly alongside environmental factors — to be behind an autism diagnosis.
“Usually, a child with autism is born to parents who don’t have autism,” Dr. Cuffman says. “It’s usually a combination of factors that leads to autism.” This is very individualized medicine, meaning that determining the cause of your child’s (or children’s) autism — whether it’s a single- or multi-gene disorder — usually requires genetic testing.
That said, we’re learning that certain conditions raise the likelihood of having a child with ASD.
The role of ADHD
For a long time, the American Psychiatric Association treated attention-deficit/hyperactivity disorder (ADHD) and ASD as conditions that were mutually exclusive: You could have ADHD or ASD, but not both.
That changed in 2015, when the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders— known as the DSM5 — came out. In the years since, scientists have been playing catch-up, trying to understand how these conditions are, and aren’t, related. Now that it’s possible to be diagnosed with both ADHD and ASD, some people embarking on parenthood are learning they were mis- or under-diagnosed as children.
According to the scientific literature, 50% to 70% of children with autism also have ADHD. The two conditions often appear together. But there’s also increasing evidence to suggest that having ADHD may raise the chances of having children with autism. A 2014 study found that mothers with an ADHD diagnosis have a two-and-a-half-times greater chance of their first child having autism.
“ADHD and ASD are both forms of neurodivergence. There are genetic linkages in regions of genes and chromosomes that connect the two,” Dr. Cuffman explains.
In Dr. Cuffman’s clinical experience, in addition to having family members with autism, ADHD is the most common familial link she sees.
Other potential risk factors
We also know that, statistically speaking, ASD is more common in children who are:
- Assigned male at birth (AMAB).
- Born extremely preterm (before 26 weeks of gestation).
But are there other conditions, like ADHD, that could help predict whether or not your child will develop ASD?
According to a 2019 study, having a family history of mental and neurological disorders may increase the likelihood of a child developing ASD. Another study found that siblings of children with autism were more likely to experience psychiatric or neurodevelopmental disorders like anxiety, schizophrenia, bipolar and tic disorders like Tourette’s syndrome. This suggests these conditions may be linked. But research on the possibility of a link between ASD and intellectual disabilities has yielded mixed results.
One thing’s for sure: If one of your children is diagnosed with ASD, you should monitor your other children closely, and get them evaluated if you have any concerns.
Early intervention is important
You’ve probably heard the saying, “Knowledge is power.” That’s especially true when it comes to ASD.
“If you have one child diagnosed with autism and have concerns about your other child, early intervention is key. They are at higher risk, so you should be talking to your primary care pediatrician and get a referral to see a developmental pediatrician earlier rather than later,” Dr. Cuffman reiterates.
The research is clear: Early intervention can make a big difference in a child’s function and quality of life. If you already have a child with ASD, you can use what you know — and the medical connections you made in your first child’s diagnostic process — to start assessment early.
Autism is nobody’s ‘fault’
All too often, when searching for answers to the kinds of questions we’re asking here, you stumble on websites that try to assign blame to one parent or another for a child’s ASD diagnosis.
As we hope we’ve made clear here, the genetics of autism are complex — and there’s nothing you could have done, or not done, to prevent your child (or children) from developing the disorder. Simply put: There is no guilt to assign.
Of course, knowing that and accepting it can be two very different things.
If you’re struggling to process your child’s ASD diagnosis, Dr. Cuffman suggests reaching out for help. “Talking to a psychologist, or other parents who’ve had similar experiences, can help you cope.” And remember: Needing support during a particularly stressful time doesn’t mean you love your child any less.
The bottom line
Autism spectrum disorder is an umbrella diagnosis. Lots of different kinds of developmental conditions can cause a child to develop autism, from single-gene disorders to a complex interplay of genetic and environmental factors.
Research shows that autism can indeed run in families. If you already have one child with ASD, you have a 1 in 5 chance of your next child developing autism. If you have more than one child with ASD, the odds of having another child with ASD are even higher. Certain conditions, like ADHD, also appear to have some predictive value.
There’s nothing a parent can do, or not do, to prevent their children from developing autism. But being aware of the complicated nature of the condition can empower you to make informed choices — and get your children the best possible care as quickly as possible.