Is Thumb Sucking a Sign of Autism? What the Research Says
Clinically reviewed by Dr. Abinaash Kaur, DDS, RCDSO, on 2026-10-19. Dr. Kaur is a general dentist in Toronto (Bloor West Village) registered with the Royal College of Dental Surgeons of Ontario.
Thumb sucking isn't on the Public Health Agency of Canada's list of autism signs, and it's common in babies and young children. One small study found it more often in autistic preschoolers, but that finding can't tell you whether your child is autistic, and only a doctor can answer that.
I'm a dentist, not a developmental specialist. I can tell you what the research on thumb sucking and autism does and doesn't say, and what the habit does to teeth. If something about your child's development worries you, the person to call is your family doctor or paediatrician.
Have no fear about asking. It's a fair question, and you're far from the only parent asking it.
Why does this question come up at all?
Sucking is a reflex babies are born with. The Canadian Dental Association describes it as a natural reflex that helps babies feel calm and secure, common in infancy and early childhood, with the need to suck usually fading by age two or three (Canadian Dental Association).
So most children do it. Some keep doing it longer than their parents expect.
A habit that hangs on, plus a search engine, is how a lot of worried parents end up typing this question late at night. The research behind it turns out to be one small study.
What did that one study find?
Researchers in Riyadh, Saudi Arabia, asked parents of 150 autistic preschoolers and 150 matched children without autism to fill in a questionnaire about oral habits. The children were three to six years old (Pakistan Journal of Medical Sciences via PubMed).
Oral habits of some kind showed up in 87.3% of the autistic children and 49.3% of the comparison group. The most common habits in the autistic group were teeth grinding at 54.7%, object biting at 44.7% and mouth breathing at 26.7% (Pakistan Journal of Medical Sciences via PubMed).
Thumb sucking wasn't in that top three. The study reports that thumb sucking, along with grinding, object biting and tongue biting, was significantly more common in the autistic group (Pakistan Journal of Medical Sciences via PubMed).
Here's the number most summaries leave out. Adding up the study's own tables, 17 of the 150 autistic children had a thumb habit, and 5 of the 150 comparison children did (Pakistan Journal of Medical Sciences full text, PMC).
Read that the other way. About nine in ten of the autistic children in the study didn't suck their thumbs at all.
Why can't that study answer your question?
Most people think a habit that shows up more often in autistic children must be a warning sign. The logic doesn't work that way.
The study started with children who were already diagnosed, then asked what habits they had. Your question runs in the opposite direction: you start with a habit and ask about autism. Those are different questions, and a study built for the first one can't answer the second.
Numbers matter here too. The 2019 Canadian Health Survey on Children and Youth found about 1 in 50 Canadian children and youth aged 1 to 17 had been diagnosed with autism (Public Health Agency of Canada). That leaves 49 in every 50 who aren't, and thumb sucking is common in young children (Canadian Dental Association).
Put those together and most thumb-sucking children you'll ever meet aren't autistic. A habit that common can't sort children into groups on its own.
There are limits on the study itself as well. It was one city, one age band, and parents' answers on a questionnaire rather than direct observation (Pakistan Journal of Medical Sciences via PubMed). When I searched for more research on thumb sucking and autism, I found little else.
What do doctors look for instead?
The Public Health Agency of Canada lists early signs that centre on communication and connection. Up to 12 months, these include little or no babbling, little or no eye contact, and seeming not to hear when spoken to directly (Public Health Agency of Canada).
By age two, the list adds things like limited interest in other children, repeating words without seeming to understand them, and trouble with back-and-forth play such as peek-a-boo (Public Health Agency of Canada).
Thumb sucking doesn't appear anywhere on that list.
I'm sharing that list so you can see where the professional focus sits, not so you can score your child against it. The same Public Health Agency of Canada page says there's no simple medical test for autism, and that a specialist is the best person to give a diagnosis (Public Health Agency of Canada).
Who should you talk to if you're worried?
Start with your family doctor or paediatrician. The Public Health Agency of Canada says to speak to your healthcare provider about an assessment if you have concerns, if you notice signs, or if a close relative such as a sibling has been diagnosed (Public Health Agency of Canada).
It also suggests ways to help the appointment along: photos, a log or diary, or short videos of what you've noticed (Public Health Agency of Canada).
You don't need a thumb habit, or any single behaviour, as a reason to ask. A gut feeling is enough to book the visit.
What I won't do is tell you your child is or isn't autistic from a dental chair. In about 25 years of seeing families in Bloor West Village, I've learned to stay in my lane on this one. My lane is the teeth.
What does thumb sucking do to teeth?
Teeth are squarely my job. The Government of Canada says prolonged thumb or finger sucking can permanently change the shape of the mouth and the position of the teeth and lips (Government of Canada).
The Canadian Dental Association's advice is to aim to stop thumb or soother sucking by age five, before the permanent teeth begin to come in, because ongoing sucking after that may affect how the teeth and jaws grow (Canadian Dental Association).
What we look for at a checkup is the bite. Do the front teeth meet? Are the upper teeth pushed forward? Is the roof of the mouth changing shape? Our post on what thumb sucking does to the bite walks through open bite, crossbite, and the other things that can sit underneath a stubborn habit.
None of that tells me anything about autism. It tells me about the bite.
When should you step in on the habit?
Pretty much never before two or three. A toddler with a thumb in their mouth is doing something most toddlers do, and our guide to baby oral health covers what matters more at that stage.
As your child gets closer to five, it's worth starting gently. The CDA notes that a soother can be taken away gradually in a way a thumb can't, which is one reason it prefers a soother when soothing is still needed (Canadian Dental Association).
Try this:
- Praise the thumb staying out, and skip the scolding when it goes in.
- Notice when it happens. Tired, bored and anxious each need a different fix.
- Pick one small goal together and mark it on a calendar.
- Bring your child in. Kids often listen to their dentist when they've tuned out a parent.
What if your child is autistic and sucks their thumb?
The dental advice stays the same. The habit gets the same gentle approach and the same age-five target, adjusted to what your child can manage.
The same Riyadh study found teeth grinding was the most common habit in its autistic group (Pakistan Journal of Medical Sciences via PubMed), so it's worth mentioning grinding at your child's checkup too. Our post on how to stop grinding teeth at night explains what we look for.
Dental visits can feel like a lot for any child. In one survey of 395 parents of autistic children in India, children who had thumb-sucking and nail-biting habits were cooperative with the dentist (International Journal of Clinical Pediatric Dentistry via PubMed). That's one survey, so I wouldn't lean on it, but it's a nice counterweight to the worry.
Some days the schedule pushes me, and even then we'll slow down for a child who needs a quieter, slower visit. Tell us what works at home, and we'll copy it.
Frequently Asked Questions
Q: Is thumb sucking a sign of autism? Thumb sucking isn't one of the signs listed by the Public Health Agency of Canada, which focuses on things like eye contact, babbling, and how a child plays and connects with others (Public Health Agency of Canada). If you're worried about your child's development, talk to your family doctor or paediatrician.
Q: Do autistic children suck their thumbs more? In one study of 150 autistic preschoolers and 150 matched children in Saudi Arabia, thumb sucking was significantly more common in the autistic group (Pakistan Journal of Medical Sciences via PubMed). Most of the autistic children in that study didn't suck their thumbs, and it's the only study of its kind I could find.
Q: How common is autism in Canadian children? The 2019 Canadian Health Survey on Children and Youth found about 1 in 50 Canadian children and youth aged 1 to 17 had been diagnosed with autism (Public Health Agency of Canada).
Q: Can a dentist tell if my child is autistic? No. The Public Health Agency of Canada says there's no simple medical test for autism and that a specialist is the best person to give a diagnosis, usually after a referral from your healthcare provider (Public Health Agency of Canada). A dentist can tell you how a habit is affecting the teeth and bite.
Q: When should my child stop sucking their thumb? The Canadian Dental Association says to aim to stop thumb or soother sucking by age five, before the permanent teeth begin to come in (Canadian Dental Association). Most children feel less need to suck by two or three.
Reviewed by Dr. Abinaash Kaur, B.Sc., DDS (University of Toronto Faculty of Dentistry), who has practised general and family dentistry in Bloor West Village for about 25 years. This article is general oral health information, not a diagnosis or a substitute for an examination, and it is not a developmental screening. Questions about your child's development belong with your family doctor or paediatrician.
Wondering whether your child's thumb habit has changed their bite? Come in and we'll check the teeth, talk through what your child is ready for, and go at their pace. We're at 750 Annette Street in Bloor West Village, serving the Junction, High Park, and Baby Point. Book a visit with The Village Dentist and we'll make a plan together.
