Stuttering and Intelligence: What IQ Research Shows About People Who Stutter

People who stutter have IQ scores in the normal range, and stuttering is not caused by either low or high intelligence. Some older clinic studies found slightly lower average scores, but large community studies of young children find little or no difference, which means a stutter tells you almost nothing about how well a person reasons.

This article covers what the research shows about IQ in people who stutter, why older studies pointed in a different direction, where the popular "stuttering is a sign of intelligence" idea comes from, how stuttering differs from a language disorder, and what an examiner should keep in mind when testing someone who stutters.

Stuttering is a speech fluency disorder marked by repeated sounds and syllables, stretched-out sounds or silent blocks in which no sound comes out at all. The diagnostic manual used in the United States calls it childhood-onset fluency disorder. It is common in early childhood and much rarer later. A random telephone survey of households in New South Wales, Australia, by Craig and colleagues found a prevalence of 0.72% across all ages, with about 1.4% of young children and 0.53% of adolescents stuttering at the time. In their 2013 review, Yairi and Ambrose concluded that the lifetime chance of ever stuttering is at least the traditional estimate of 5% and possibly closer to 8%.

The best modern IQ evidence comes from community cohorts, which follow everyone in a population and do not depend on who gets referred to a clinic:

• The Early Language in Victoria Study: Reilly and colleagues followed 1,619 Australian children from infancy. By age 4, 11.2% had started to stutter. On standardized tests at age 4, children who stuttered averaged 106.5 on nonverbal cognition against 103.9 for children who did not, and 105.0 against 99.6 on language. Both groups were in the average range, and the small differences favored the children who stuttered.

• The follow-up at age 7: Kefalianos and colleagues found that 65% of those children had recovered by age 7, and there were no evident differences in nonverbal cognition between children who recovered and those who still stuttered.

• The Generation R Study in Rotterdam: Koenraads and colleagues reported that 97 of 2,211 children (4%) had a history of stuttering by about age 9. Nonverbal IQ, measured earlier with a standard Dutch test, averaged 104.9 in fluent children, 103.8 in children who still stuttered and 102.6 in children who had recovered. The differences were not statistically significant.

A "standard deviation" on most IQ tests is 15 points, so gaps of 1 to 3 points are a small fraction of one. For practical purposes, children who stutter and children who do not have the same spread of reasoning ability.

The idea that people who stutter score lower on IQ tests is not invented. In a review of the stuttering literature, Howell described the 1964 study by Andrews and Harris, in which 80 children who stuttered scored lower than controls on the Wechsler scales, on both verbal and nonverbal subtests. In that study, 31 of the 80 children who stuttered had IQs below 90, compared with 13 of 90 controls. A few other older studies reported similar gaps.

Howell also noted that this pattern "is contradicted by more recent work." In one later sample described in his review, Yairi and Ambrose found that young children who stuttered scored 13 to 22 points above the test norms on a nonverbal scale, and another report found that children in treatment came from above-average backgrounds. Neither finding means stuttering raises IQ. The likeliest explanation for all of these swings is sampling. Who ends up in a clinic or a research study depends on referral habits and family resources, and on which children have other difficulties alongside the stutter. A low average in one clinic and a high average in a university study can both be artifacts of who walked through the door.

A popular claim online is that stuttering happens because a child's mind runs faster than their mouth, so stuttering must signal a high IQ. The evidence does not support that.

The claim has a small grain of truth behind it. In the Victorian cohort, stuttering onset was linked with higher vocabulary scores at age 2 and with higher maternal education, and the children who stuttered had slightly stronger language at age 4. Reilly's team suggested that onset "seems to be associated with rapid growth in language development." A burst of new words and longer sentences may put extra load on a young child's speech system at the moment stuttering tends to begin.

That is a long way from a high IQ. The advantages were a few points, the predictors explained only 3.7% of the variation in who started to stutter, and nonverbal scores were essentially average. Stuttering is found across the whole range of ability, and no study shows that people who stutter, as a group, have unusually high IQs. The same goes for lists of famous people said to have stuttered: they show only that stuttering is compatible with success.

A person who stutters usually knows exactly what they want to say. The difficulty lies in producing the words smoothly. That separates stuttering from a language disorder, in which vocabulary and grammar themselves lag behind. Our article on how developmental language disorder is diagnosed covers that separate condition. The two can co-occur in the same child, and an evaluation should check for both.

The causes of stuttering appear to be largely biological. In 2010, Kang and colleagues identified mutations in GNPTAB, GNPTG and NAGPA, three genes in a pathway that directs enzymes to the lysosome, in some families and individuals with persistent stuttering. The finding supports a biological basis for the speech problem itself.

What does often travel with persistent stuttering is social anxiety. In a study of 92 adults seeking speech treatment, Iverach and colleagues found 16- to 34-fold higher odds of meeting criteria for social phobia than in matched controls. Anxiety matters for testing, because a nervous examinee may say less and answer more cautiously.

Most IQ subtests do not reward speed of speech, but several features of a test session can still work against someone who stutters.

• Oral answers on verbal subtests: Vocabulary, similarities and general knowledge items are answered out loud. A person who swaps a hard-to-say word for an easier one may give a less precise answer and lose a point. The American Speech-Language-Hearing Association notes that some people who stutter "substitute and/or omit words or use circumlocution to avoid stuttering." Our guide to the verbal comprehension index explains which subtests feed that score.

• Examiner patience: Long blocks can tempt an examiner to move on or prompt early. A careful examiner waits for the full answer and scores its content.

• Comparing verbal and nonverbal scores: If nonverbal reasoning is clearly higher than verbal scores, the examiner should consider whether speaking demands, rather than knowledge, explain the gap.

• Accommodations: Extra time for oral responses and written answers where the test allows are the usual adjustments. Our article on testing accommodations explains how these work without changing what a test measures.

A speech-language pathologist is the right professional to assess the stutter itself. A psychologist who gives an IQ test should ideally know the person's fluency history before interpreting verbal scores.

Stuttering usually changes a great deal in childhood. Most onset happens before age 5, and Yairi and Ambrose estimated that around 80% of children who start to stutter eventually recover. Recovery is not quick, though: in the Victorian cohort only 6.3% of children had recovered within 12 months of onset. There is no evidence that IQ rises or falls as a stutter comes and goes, and community data show no difference in nonverbal ability between children who recover and children who persist.

No. Community studies find that children who stutter score in the average range on nonverbal and language tests, close to or slightly above their fluent peers.

No good evidence shows that. Stuttering onset is linked with rapid early language growth in some children, but the differences are small and people who stutter span the whole range of ability.

It can lower a verbal subtest score if the person avoids words or the examiner rushes them. Comparing verbal and nonverbal scores, and testing with patience, helps separate fluency from reasoning.

No. Stuttering is a fluency disorder that affects how smoothly words come out. A language disorder affects understanding or building sentences, although a child can have both.

A speech-language pathologist assesses the stutter and checks language skills. If there are learning concerns, a psychologist who knows the child's fluency history can give a cognitive assessment.

Stuttering and intelligence are largely separate. Community cohorts in Australia and the Netherlands find that children who stutter have average nonverbal ability, and older reports of lower scores most likely reflected who was referred to clinics. The "sign of intelligence" idea grows from a small link with early language growth, and no study shows higher IQs. When a person who stutters is tested, verbal scores deserve a careful look next to nonverbal ones. Readers curious about their own reasoning ability, measured without any speaking at all, can take the RIOT IQ test.

1. Craig, A., Hancock, K., Tran, Y., Craig, M., & Peters, K. (2002). Epidemiology of stuttering in the community across the entire life span. Journal of Speech, Language, and Hearing Research, 45(6), 1097-1105. [doi.org/10.1044/1092-4388(2002/088)](. doi.org

2. Yairi, E., & Ambrose, N. (2013). Epidemiology of stuttering: 21st century advances. Journal of Fluency Disorders, 38(2), 66-87. [doi.org/10.1016/j.jfludis.2012.11.002](. doi.org

3. Reilly, S., Onslow, M., Packman, A., Cini, E., Conway, L., Ukoumunne, O. C., Bavin, E. L., Prior, M., Eadie, P., Block, S., & Wake, M. (2013). Natural history of stuttering to 4 years of age: A prospective community-based study. Pediatrics, 132(3), 460-467. [doi.org/10.1542/peds.2012-3067](. doi.org

4. Kefalianos, E., Onslow, M., Packman, A., Vogel, A., Pezic, A., Mensah, F., Conway, L., Bavin, E., Block, S., & Reilly, S. (2017). The history of stuttering by 7 years of age: Follow-up of a prospective community cohort. Journal of Speech, Language, and Hearing Research, 60(10), 2828-2839. [doi.org/10.1044/2017_JSLHR-S-16-0205](. doi.org

5. Koenraads, S. P. C., van der Schroeff, M. P., van Ingen, G., Lamballais, S., Tiemeier, H., Baatenburg de Jong, R. J., White, T., Franken, M. C., & Muetzel, R. L. (2020). Structural brain differences in pre-adolescents who persist in and recover from stuttering. NeuroImage: Clinical, 27, 102334. [doi.org/10.1016/j.nicl.2020.102334](. doi.org

6. Howell, P. (2007). Signs of developmental stuttering up to age eight and at 12 plus. Clinical Psychology Review, 27(3), 287-306. [doi.org/10.1016/j.cpr.2006.08.005](. doi.org

7. Reilly, S., Onslow, M., Packman, A., Wake, M., Bavin, E. L., Prior, M., Eadie, P., Cini, E., Bolzonello, C., & Ukoumunne, O. C. (2009). Predicting stuttering onset by the age of 3 years: A prospective, community cohort study. Pediatrics, 123(1), 270-277. [doi.org/10.1542/peds.2007-3219](. doi.org

8. Kang, C., Riazuddin, S., Mundorff, J., Krasnewich, D., Friedman, P., Mullikin, J. C., & Drayna, D. (2010). Mutations in the lysosomal enzyme-targeting pathway and persistent stuttering. New England Journal of Medicine, 362(8), 677-685. [doi.org/10.1056/NEJMoa0902630](. doi.org

9. Iverach, L., O'Brian, S., Jones, M., Block, S., Lincoln, M., Harrison, E., Hewat, S., Menzies, R. G., Packman, A., & Onslow, M. (2009). Prevalence of anxiety disorders among adults seeking speech therapy for stuttering. Journal of Anxiety Disorders, 23(7), 928-934. [doi.org/10.1016/j.janxdis.2009.06.003](. doi.org

10. American Speech-Language-Hearing Association. (n.d.). Fluency disorders [Practice portal]. [asha.org/practice-portal/clinical-topics/fluency-disorders](. asha.org

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