Is dyslexia a mild form of autism?

Dyslexia and autism spectrum disorder (ASD) are two distinct neurodevelopmental disorders that have some overlapping characteristics but also key differences. While there has been some speculation that dyslexia may represent a mild form of autism, the current research evidence suggests they are separate conditions.

Summary of Key Points

• Dyslexia is a learning disorder characterized by difficulties with reading and spelling, while autism spectrum disorder (ASD) is a developmental disability mainly characterized by challenges with social communication and interaction, as well as restricted/repetitive behaviors.
• Individuals with dyslexia typically have average or above average intelligence, while intellectual disability can co-occur with ASD.
• There are some similarities between dyslexia and ASD such as difficulties with sensory processing, motor skills, attention, and anxiety. However, key differences set the conditions apart.
• Both dyslexia and ASD have been linked to atypical brain connectivity and structure, but the specific neural patterns differ between the two.
• Genetic studies indicate some shared genetic risk factors between dyslexia and ASD, but also distinct genetics underlying each disorder.
• While dyslexia and ASD may co-occur in some individuals, most evidence indicates they are distinct disorders with overlapping behavioral features rather than dyslexia being a mild variant of ASD.

What is Dyslexia?

Dyslexia is a common learning disorder characterized by difficulties with accurate and/or fluent word reading and poor spelling. It is estimated to affect around 5-10% of the population.¹ The core challenge for individuals with dyslexia is a deficit in phonological processing – the ability to detect, segment, and manipulate the individual sounds (phonemes) in spoken words.²

Difficulties with phonological processing then impact reading fluency and the ability to decode new written words. Individuals with dyslexia may struggle with rhyming, learning letter-sound correspondences, pronouncing multisyllabic words, and rapid word retrieval or naming speed.³ As a result, reading is often slow, inaccurate, and effortful, despite having adequate intelligence and educational opportunities.

In addition to reading and spelling struggles, dyslexia is associated with potential weaknesses in auditory and/or visual processing, motor skills, memory, attention, and organizational abilities.⁴ However, dyslexia refers specifically to the phonological deficit and is distinct from intellectual disabilities, sensory impairments, lack of educational opportunity, or other factors leading to poor reading.

Key Characteristics of Dyslexia

• Difficulties with phonological processing
• Challenges learning letter-sound relationships
• Poor spelling skills
• Slow, inaccurate reading
• Weaknesses in short-term/working memory
• Difficulties with rapid naming/word retrieval

What is Autism Spectrum Disorder?

Autism spectrum disorder (ASD) is a developmental disability defined by persistent challenges with social communication and social interaction as well as restricted, repetitive patterns of behavior, interests, or activities.⁵ The term “spectrum” refers to the wide diversity in the severity of symptoms, skills, and support needs among people with ASD.

Some of the hallmark characteristics of ASD include:⁶

• Deficits in social-emotional reciprocity (e.g. abnormal social approach, failure of normal back and forth conversation)
• Impairments in nonverbal communicative behaviors used for social interaction
• Difficulties developing and maintaining relationships
• Stereotyped or repetitive motor movements, use of objects, or speech
• Insistence on sameness, rigid routines, or ritualized patterns
• Highly restricted, fixated interests
• Hyper- or hyporeactivity to sensory input

The onset of ASD symptoms occurs in early childhood, typically before the age of 3. Intellectual disability co-occurs with ASD in around 31% of individuals, while 25% have average to above average intelligence.⁷ Boys are diagnosed with ASD around 4 times more often than girls.

Key Characteristics of Autism Spectrum Disorder

• Social communication/interaction challenges
• Restricted/repetitive behaviors
• Sensory processing differences
• Sameness and routinized behaviors
• Focused interests and fixations

Overlap Between Dyslexia and Autism

There are certainly some overlapping behavioral and cognitive characteristics between dyslexia and ASD. Both conditions involve atypical neural connectivity and development, arising from genetic and environmental factors.⁸

Some common overlaps include:

• Sensory processing differences – Individuals with dyslexia or ASD may have enhanced perception in some sensory domains paired with deficits in others. There may be atypical sensory reactivity.
• Motor deficits – Challenges with gross and fine motor skills, balance, coordination, and motor planning are common in both conditions.
• Attention deficits – Sustained, divided, and selective attention weaknesses frequently co-occur with both dyslexia and ASD.
• Working memory challenges – Short term memory and manipulation of information deficits are typical in both disorders.
• Anxiety – High rates of anxiety disorders and symptoms frequently accompany dyslexia and ASD.

These overlapping traits likely stem from common underlying neural mechanisms. However, they manifest through different core deficits specific to each disorder.

Key Differences Between Dyslexia and Autism

Despite some overlapping characteristics, there are key differences that set dyslexia and ASD apart as distinct conditions:

1. Reading and language abilities

By definition, individuals with dyslexia struggle with accurate and fluent reading and spelling. Phonological processing problems persist into adulthood.¹ In contrast, many individuals with ASD have intact reading and language abilities, especially those with normal intelligence and verbal skills. While language delays or impairments can co-occur with ASD, they are not core defining features.

2. Social communication and interaction

Social communication and interaction deficits are central to ASD, but not typically impaired in dyslexia. Individuals with dyslexia do not exhibit the key social challenges that characterize ASD such as lack of reciprocity, nonverbal communication deficits, and difficulty developing peer relationships.⁹

3. Restricted and repetitive behaviors

Restricted interests, repetitive movements, and insistence on routines and patterns are hallmark symptoms of ASD. These behaviors are not defining or characteristic features of dyslexia.

4. Age of onset

The signs of ASD emerge in early childhood, by around age 3. In contrast, the reading and spelling difficulties associated with dyslexia tend to become more apparent later, when children begin learning to read in school.

5. Cognitive profile

On average, individuals with dyslexia have cognitive abilities in the normal or above average range. Intellectual disability and dyslexia can co-occur but not typically.¹⁰ Conversely, around 31% of individuals with ASD also have intellectual impairment.⁷

Brain Differences

Neuroimaging research indicates some similarities but also key differences in the brain characteristics associated with dyslexia compared to ASD.

In dyslexia, core deficits seem to originate in left hemisphere posterior brain regions important for reading and phonology, showing both functional and anatomical differences compared to typical readers.¹¹

In contrast, ASD has been associated with abnormalities in cortical connectivity networks across multiple brain regions, with differences in frontal, temporal, and limbic areas involved in social-emotional processing.¹²

While both conditions involve atypical connectivity, the specific neural patterns differ based on the distinct cognitive-behavioral profiles of each disorder.

Genetic Factors

Research on the genetics of dyslexia and ASD provides some mixed insights on potential overlap between the conditions.

Family and twin studies show that both disorders have a strong hereditary component, with genetic factors explaining 40-60% of cases.^13,14 Some candidate risk genes, such as ROBO1, DCDC2, and KIAA0319, have been associated with both dyslexia and ASD.¹⁵

However, genome-wide studies indicate that the majority of common genetic variants overlap between written language disorders (including dyslexia) and ASD is relatively small. Each condition has distinct genetic influences that account for most of the heritability.¹⁶

Overall, the genetic evidence suggests some shared genetic risk between dyslexia and ASD, but substantial distinct and disorder-specific genetic factors as well.

Comorbidity Between Dyslexia and Autism

Some individuals with dyslexia also meet diagnostic criteria for ASD, and vice versa. However, most prevalence estimates indicate the comorbidity rates are relatively low:

• Only around 2-5% of children with dyslexia also have ASD.

¹⁷

• Approximately 5-10% of individuals with ASD also have dyslexia.

¹⁸

This suggests that in most cases dyslexia and ASD do not directly overlap, but rather occur concurrently in a small subset of individuals. This may be partly explained by shared genetic risk factors.

Importantly, a dual diagnosis requires that the specific diagnostic criteria for both dyslexia and ASD are met independently. Merely having overlapping traits (e.g. sensory issues) does not warrant a dual diagnosis if the core features of each disorder are not present.

Can Dyslexia Be Considered a Mild Form of Autism?

Based on the evidence, dyslexia does not appear to represent a mild variant or subset of ASD. Key points supporting this include:

• The distinct cognitive-behavioral profiles of each disorder, with dyslexia characterized by written language deficits and ASD characterized by social-communication/interaction and restricted, repetitive behaviors.
• Differences in brain structure and function linked to dyslexia vs ASD.
• Largely distinct genetic factors underlying dyslexia vs ASD, with only small overlaps in heritability.
• Low rates of comorbidity, indicating most individuals with dyslexia do not have symptoms of ASD and vice versa.

Rather than a single disorder with mild vs severe variants, research supports dyslexia and ASD as two distinct neurodevelopmental conditions that can co-occur in some cases, but have separate underlying causes.

Conclusions

In summary, while dyslexia and autism spectrum disorder share some similar behavioral and cognitive characteristics, key differences support them being defined as distinct disorders rather than dyslexia reflecting a mild form of autism.

Both conditions involve atypical brain connectivity and structure arising from genetic and environmental factors. And comorbidity does occur in a small subset of cases. However, dyslexia and ASD have core defining features that differ in onset, severity, cognitive profile, and underlying neural and genetic correlates.

Ongoing research on the boundaries and overlap between neurodevelopmental disorders will continue to provide insights into the relationship between conditions like dyslexia and ASD. But current evidence suggests they are best conceptualized as distinct disorders that can co-occur rather than existing on a single continuum.

References

1. Peterson, R.L., & Pennington, B.F. (2012). Developmental dyslexia. The Lancet, 379(9830), 1997-2007.
2. Vellutino, F. R., Fletcher, J. M., Snowling, M. J., & Scanlon, D. M. (2004). Specific reading disability (dyslexia): What have we learned in the past four decades?. Journal of child psychology and psychiatry, 45(1), 2-40.
3. Lyon, G. R., Shaywitz, S. E., & Shaywitz, B. A. (2003). A definition of dyslexia. Annals of dyslexia, 53(1), 1-14.
4. Norton, E. S., Beach, S. D., & Gabrieli, J. D. (2015). Neurobiology of dyslexia. Current opinion in neurobiology, 30, 73-78.
5. American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (DSM-5®). American Psychiatric Pub.
6. Lord, C., Elsabbagh, M., Baird, G., & Veenstra-Vanderweele, J. (2018). Autism spectrum disorder. The Lancet, 392(10146), 508-520.
7. Maenner, M. J., Shaw, K. A., Baio, J., et al. (2020). Prevalence of autism spectrum disorder among children aged 8 years – autism and developmental disabilities monitoring network, 11 sites, United States, 2016. MMWR Surveillance Summaries, 69(4), 1–12.
8. Gilger, J. W., & Kaplan, B. J. (2001). Atypical brain development: A conceptual framework for understanding developmental learning disabilities. Developmental neuropsychology, 20(2), 465-481.
9. Whitehouse, A. J., & Bishop, D. V. (2008). Do children with autism ‘switch off’to speech sounds? An investigation using event-related potentials. Developmental science, 11(4), 516-524.
10. Snowling, M. J., & Hulme, C. (2012). Annual research review: the nature and classification of reading disorders–a commentary on proposals for DSM‐5. Journal of Child Psychology and Psychiatry, 53(5), 593-607.
11. Norton, E. S., Beach, S. D., & Gabrieli, J. D. (2015). Neurobiology of dyslexia. Current opinion in neurobiology, 30, 73-78.
12. Hull, L., Petrides, K. V., Allison, C., Smith, P., Baron-Cohen, S., Lai, M. C., & Mandy, W. (2017). “Putting on My Best Normal”: Social Camouflaging in Adults with Autism Spectrum Conditions. Journal of autism and developmental disorders, 47(8), 2519–2534.
13. Pennington, B. F., & Olson, R. K. (2005). Genetics of dyslexia. The science of reading: A handbook, 453-472.
14. Tick, B., Bolton, P., Happé, F., Rutter, M., & Rijsdijk, F. (2016). Heritability of autism spectrum disorders: a meta‐analysis of twin studies. Journal of Child Psychology and Psychiatry, 57(5), 585-595.
15. Poelmans, G., Buitelaar, J. K., Pauls, D. L., & Franke, B. (2011). A theoretical molecular network for dyslexia: integrating available genetic findings. Molecular psychiatry, 16(4), 365-382.
16. Gialluisi, A., Andlauer, T. F., Mirza-Schreiber, N., Moll, K., Becker, J., Hoffmann, P., … & Schulte-Körne, G. (2020). Genome-wide association study reveals new insights into the heritability and genetic correlations of developmental dyslexia. Translational psychiatry, 10(1), 1-15.
17. Reid, G. (2016). Dyslexia: A practitioner’s handbook. John Wiley & Sons.
18. Goldstein, S., Ozonoff, S., Cook, A., & Clark, E. (2009). Autism spectrum disorders. Toward a unified theory of development: Connectionism and dynamic systems theory reconsidered, 188-213.