Do dyslexia and stuttering share a processing deficit?

Findings from a 2019 study by a student at the University of Birmingham.

Earlier this year we put up a request by Mahmoud Elsherif, a PhD psychology student from the University of Birmingham. Mahmoud was looking for people to take part in his study into the reading processes of people with dyslexia and people who stammer.

With the study now complete, he wrote a report on the findings, which you can read below, in Mahmoud's own words. There's a short summary if want a quick read, and a more detailed report below that. 

Mahmoud hopes the findings will go towards developing more effective therapy for people with speech and language difficulties. 


Mahmoud said, "Although dyslexia and stammering differ, recent research (Arndt and Healy, 2001, Blood et al., 2003) has shown that stammering may be related to dyslexia. Both groups have difficulties in processing speech sounds within a word.

'Based on these findings, we set out to explore whether people with dyslexia and people who stammer share a difficulty and assess how common childhood stammering is in people with dyslexia, together with the dyslexia profiles in people who stammer.

'We tested 84 people without a diagnosis of dyslexia and stammering, 50 people with dyslexia and 30 people who stammer. We measured behaviour that assessed things such as number recall and repeating non-words.

'We found that both groups did not differ in any phonological measure. In addition, 34% of people with dyslexia had stammered during childhood and 50% people who stammer fit the dyslexia profile. In short, dyslexia and stammering are more similar than once presumed and both conditions share a phonological difficulty."

Detailed report


Depending on how it is diagnosed, dyslexia occurs in 7-10% of the population, while stammering occurs in 1% of the population. Both conditions may appear different from one another, as dyslexia focuses mainly on literacy difficulties, whereas stammering is a difficulty with the production of speech. 

If both conditions are independent, dyslexia and stammering should occur together in at least 7 out of 10,000 people. However, 30-40% of people who stammer have been identified with a difficulty in processing and pronouncing combinations of sounds in words such as dyslexia (Arndt & Healy, 2001; Ardila et al., 1994; Blood et al., 2003). Given these numbers, both conditions highlight interesting similarities, yet no research has compared them directly. I wanted to investigate this in my study.

Aim of the study

The underlying causes of stammering and dyslexia remain unclear, and by comparing these conditions, we could discover which difficulties they do and do not share. One arguable overlap found in both conditions is a phonological (i.e. sound) processing difficulty, for instance, the word CAT is made up of three phonemes (units of sound that distinguish one word from another :'k', 'ae' and 't' (dyslexia research: e.g. Peterson & Pennington, 2015; Snowling et al., 2018; stammering research: Anderson, Wagovich & Hall, 2006; Pelczarski, 2011), but it is unclear whether the same phonological difficulty is shared within both conditions. The aim of this study was to assess the extent to which people who stammer fit the dyslexia profile, together with the proportion of people with dyslexia who stammered during childhood but recovered.


Our volunteers were given a questionnaire asking whether they had dyslexia or stammered during childhood. If a person with dyslexia had stammered during childhood, they were asked to verify this statement with their certified educational report. In addition, several tasks to assess their phonological processes, reading ability, vocabulary and non-verbal intelligence were used. These included silently reading a string of letters on screen and indicating whether they are a word or not; naming the string of letters; plus easy tests for spelling, non-verbal reasoning and short-term memory among many other tests.

For the short-term memory, you would have to repeat numbers in increasing length. For instance, you hear the digits 7704, you have to repeat it in the same order. Next, you would be given 77041 and repeat them in the same order. For spelling, you would hear a word and write down the spelling.  For non-verbal reasoning, you have to connect a panel to the pattern being shown. In addition, they were given a piece of text to read and were asked to answer questions related to the text. Finally, we gave participants a checklist, in which people were asked to recognise and tick real book titles and authors, while ignoring fake ones.


We used the phonological measures to assess whether our people with dyslexia were diagnosed with dyslexia. We found that 34% of the 50 people with dyslexia had stammered during childhood, compared to only 1% of the 84 people who had no diagnosis of dyslexia or stammering. 

Following this, we split our group with dyslexia, based on the phonological measures, into those who are the most severe and those who are mild in terms of their diagnosis of dyslexia. This was affected by the severity of dyslexia, such that people with more severe dyslexia were also more likely to have stammered: three out of 20 people (i.e. 15%) with mild dyslexia had stammered during childhood, while the percentage more than doubled in the 30 people with severe dyslexia (47% with childhood stammering).

In addition, for people who stammer, we had used the same criteria from the phonological measures to assess whether they would fit the diagnosis of dyslexia when considering people with a current stammer who had never been diagnosed with dyslexia, 50% fitted the dyslexia profile. This did not occur as a result of poor vocabulary or reading ability, as both groups were similar to one another. This shows that dyslexia and stammering have a lot in common. However, and interestingly, we found that people who stammer recognised around 50% of books and authors compared to those who did not stammer and those with dyslexia (both approximately 30%).


Our findings highlight some important avenues for future research, such as identifying the similarities and differences between both conditions in terms of reading and speech.

In our study, findings from a number of individual tests showed that people who stammer can have unusually strong reading abilities. For example, people who stammer recognised around 50% of books and authors compared to those who do not stammer and those with dyslexia (both approximately 30%). This suggests that people who stammer read more, which in turn leads to improvements in reading skills such as greater vocabulary and better reading comprehension. To quote stammering researcher Paul Brocklehurst, these are “hidden strengths” that are not often highlighted and but require attention. These strengths deserve more awareness, responding to the negativity that stammering tends to receive.

One limitation to consider is that in comparison to the people without either dyslexia or stammering, people with dyslexia provided a certified educational report. Also, the latter group are regularly informed about their speech and language difficulties that and may be more aware of their stammering during childhood. This may underestimate the childhood stammering value for people without either dyslexia or stammering. Also, we need to assess how stammering affects language processing over development (i.e. from childhood to adulthood).

This research has potential for positive outcomes in the real-world, for example interventions for people with dyslexia and people who stammer could inform each other to provide further support for both groups. This could, in turn, reduce any negative influences on an individual’s mental wellbeing, including lowering the risk of social anxiety and suicide (Boyes, Leitao, Claessen, Badcock & Nayton, 2019; Craig & Tran, 2014). This research could also lead to a better understanding of a phonological impairment in these groups in order to target effective interventions to support both populations. 

The set of materials for this study, including a pre-print of the manuscript, can be found on the Open Science Framework's website.


Anderson, J. D., Wagovich, S. A., & Hall, N. E. (2006). Nonword repetition skills in young children who do and do not stutter. Journal of fluency disorders, 31(3), 177-199.
Arndt, J., & Healey, E. C. (2001). Concomitant disorders in school-age children who stutter. Language, Speech, and Hearing Services in Schools. 32, 68-78
Ardila, A., Bateman, J., Niño, C. R., Pulido, E., Rivera, D. B., & Vanegas, C. J. (1994). An epidemiologic study of stuttering. Journal of Communication Disorders, 27(1), 37-48.
Blood, G. W., Ridenour Jr, V. J., Qualls, C. D., & Hammer, C. S. (2003). Co-occurring disorders in children who stutter. Journal of Communication Disorders, 36(6), 427-448.
Boyes, M. E., Leitão, S., Claessen, M., Badcock, N. A., & Nayton, M. (2019). Correlates of externalising and internalising problems in children with dyslexia: An analysis of data from clinical casefiles. Australian Psychologist. 
Brocklehurst, P. (2014). The hidden strengths of people who stutter.
Craig, A., & Tran, Y. (2014). Trait and social anxiety in adults with chronic stuttering: Conclusions following meta-analysis. Journal of fluency disorders, 40, 35-43.
Pelczarski, K. (2011). Phonological processing abilities of adults who stutter (Doctoral dissertation, University of Pittsburgh).
Peterson, R. L., & Pennington, B. F. (2015). Developmental dyslexia. Annual review of clinical psychology, 11, 283-307.
Snowling, M. J., Gooch, D., McArthur, G., & Hulme, C. (2018). Language skills, but not frequency discrimination, predict reading skills in children at risk of dyslexia. Psychological science, 29(8), 1270-1282.

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Tayo & Bhupinder
A speaker on stage at STAMMAFest 2023

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