What is dyslexia?


There are different ways of defining dyslexia. Some definitions are used in research, others in medical diagnosis, others in civil rights law, and still others in educational evaluations or intervention plans. 

Despite differences in wording or emphasis, they are the same at heart. Understanding models of reading and sight words helps put dyslexia, whatever definition you use, in perspective. 

As a way to demystify dyslexia, Hugh Catts (2018) suggests that a basic definition of dyslexia is unexpected trouble recognizing words while reading, focusing on measurable skills rather than underlying causes.

Dyslexia Connection: In the graphic below, individuals with dyslexia are found in the upper left quadrant, with strong language comprehension, but weak decoding skills. Individuals with dyslexia who also have language comprehension difficulties are found in the lower left quadrant.

Figure 1. The relationship between decoding and language comprehension (Catts, 2018).

IDA and NIH Definitions of Dyslexia

One of the most widely used definitions comes from the International Dyslexia Association (IDA Editorial Contributors, 2018), Wisconsin 2019 Act 86 and Wisconsin's Informational Guidebook on Dyslexia and Related Conditions

“Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.”

Another comes from the National Institutes of Health (National Institute of Neurological Disorders and Stroke, 2019):

“Dyslexia is a brain-based type of learning disability that specifically impairs a person's ability to read. These individuals typically read at levels significantly lower than expected despite having normal intelligence. Although the disorder varies from person to person, common characteristics among people with dyslexia are difficulty with phonological processing (the manipulation of sounds), spelling, and/or rapid visual-verbal responding. In individuals with adult onset of dyslexia, it usually occurs as a result of brain injury or in the context of dementia; this contrasts with individuals with dyslexia who simply were never identified as children or adolescents. Dyslexia can be inherited in some families, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia.”

The Diagnostic and Statistical Manual (DSM-5 315.00) from the American Psychiatric Association discusses dyslexia under the Specific Learning Disorders, saying:

“Dyslexia is an alternative term used to refer to a pattern of learning difficulties characterized by problems with accurate or fluent word recognition, poor decoding, and poor spelling abilities.” 

Dyslexia is also defined in federal law. Public Law 115-391, "The First Step Act," provides the following definition:

"The term 'dyslexia' means an unexpected difficulty in reading for an individual who has the intelligence to be a much better reader, most commonly caused by a difficulty in the phonological processing (the application of the individual sounds of spoken language), which affects the ability of an individual to speak, read, and spell."

A closer look at each part of the definition from the IDA highlights similarities among definitions and clarifies understanding of dyslexia:

Specific learning disability (SLD) 

Neurobiological in origin

Difficulties with accurate and/or fluent word recognition

Deficit in spelling and decoding abilities

Deficit in the phonological component of language 

Often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction

Figure 2. Adapted from Overcoming Dyslexia (Shaywitz, 2003).

Secondary consequences

What Does it Mean To Be “At Risk for Dyslexia?”

Common definitions of dyslexia focus on a child having trouble with word recognition and spelling. Therefore, younger children who aren't expected to be able to read and spell yet would not fit the definition even if brain imaging showed a structure or pattern typical of a dyslexic brain.

If a child aged 3 to 5 has a family history of dyslexia, or if early screening shows weakness in pre-reading skills, such as age-appropriate phonemic awareness, phonological memory, automatized rapid naming, or letter recognition, the child should be considered “at risk” of developing a reading or spelling problem that could later be classified as dyslexia.

A child aged three to five is in the critical stage for intervention and prevention. Intervention should never be delayed until reading failure occurs. Early evidence-based intervention is an opportunity to prevent or significantly lessen future reading and spelling difficulties.

A child with an affected parent has a 40-60% risk of developing dyslexia. The risk of developing dyslexia is increased when other family members are also affected.

Journal of Medical Genetics, 2007

Causes of Dyslexia

Dyslexia is brain-based and has a genetic component. Dyslexia runs in families. About 40% of siblings of children with dyslexia will also have the condition, and almost 50% of the children of a parent with dyslexia will be affected. If both parents have dyslexia, 75% of their children likely will be impacted. 

Tracking the genetics of dyslexia is complicated because more than one gene is involved, and environment also plays a role in the development of the disability. Studies on twins have shown that about 30% of the risk for dyslexia comes from a child’s early environment, with poverty and parent education being risk factors. For a child with a genetic predisposition, the early literacy environment and quality of instruction are especially critical (Dorta, 2021).

The inherited genes impact both the structure and functioning of the areas and pathways of the brain that are used for reading. With the development of brain imaging technology in the 1990s, scientists have been able to demonstrate that dyslexia is brain-based. The brain models below represent the normal activation patterns of the reading brain compared to the “neural signature” of the dyslexic brain engaged in the same task. 

Myths About Dyslexia

Figure 3. From The Yale Center for Dyslexia & Creativity, originally from Overcoming Dyslexia (Shaywitz, 2003).

The efficient processors of written language are found in the left hemisphere of the typically-developing brain. These parts of the brain include areas that process the visual aspects of written symbols, areas that connect symbols with speech sounds, and areas that connect words with meaning. 

The back (red and yellow) areas are more heavily activated in reading than the front (green) area. Beginning readers slowly analyze words in the red area with some assistance from the green area. After a word is analyzed correctly several times, its spelling, pronunciation, and meaning are stored permanently in the yellow “word form” area, where maturing readers can recognize it instantly. A skilled reader can effortlessly and quickly recognize most words in the word form area (Shaywitz, 2003).

On the other hand, individuals with dyslexia experience disruption of the left rear pathways, and show under-activation of the red and yellow areas. Over time, they rely more and more heavily on the green area, along with some less efficient right-hemisphere areas, to figure out words. This pattern of reading, though it may provide some degree of accuracy, does not lead to the effortless and instantaneous word recognition that is so important for reading comprehension and motivation (Shaywitz, 2003).

The good news is that a group of studies has presented strong evidence that early, evidence-based intervention can improve reading performance and also change dyslexic brain activation patterns to more closely resemble non-impaired functioning. Typical of this line of research is a 2003 study by Shaywitz et al., in which struggling readers were provided with a phonologically-based reading intervention over the course of a year. At the end of the intervention, fMRI imaging showed increased activation in the red and green areas of the left brain, and students were reading more accurately and fluently. Moreover, a year after the intervention ended, the same students showed activation in the critical yellow word form area of the brain, and less activation in the right hemisphere. Struggling readers who had received a non-phonologically-based intervention did not show these brain changes. Study authors concluded: 

[T]hese data demonstrate that an intensive evidence-based (phonologic) reading intervention brings about significant and durable changes in brain organization, so that brain activation patterns resemble those of typical readers, with the appearance of the left occipitotemporal area and improvement in reading fluency. These data have important implications for public policy regarding teaching children to read: the provision of an evidence-based reading intervention at an early age improves reading fluency and facilitates the development of those neural systems that underlie skilled reading.

Are Word Recognition Problems Always a Sign of Dyslexia?

Word recognition problems do not all emerge from dyslexia. Children may struggle to acquire the foundational skills of reading for many other reasons, both personal (poor vision or hearing, short attention span, low global intelligence, slow cognitive development, lack of motivation, etc.) and environmental (low literacy home environment, erratic school attendance, nutritional status, trauma, poor instruction, etc.). What distinguishes dyslexia from these related conditions are the language-related aspects of the reading problem and its genetic and neurobiological roots. 

ALL children with reading difficulties deserve effective intervention. Children who struggle with automatic word recognition, regardless of the reason, need the explicit and systematic instruction in the alphabetic code recommended by the Report of the National Reading Panel in 2000 and the DSM-5 manual of the American Psychiatric Association (Elliott and Grigorenko, 2014). 

Uncovering the underlying causes of the word recognition problem in dyslexia is not necessary to begin effective intervention, but may be helpful in forecasting the need for a more intensive and extensive intervention than might be needed for other children who are exhibiting similar symptoms.

What More Do We Know About Dyslexia?

Characteristics of Dyslexia at Different Ages

Characteristics of Dyslexia at Different Ages - WI Dyslexia Roadmap.pdf

Podcasts & Videos About Dyslexia

Additional Resources About Dyslexia


115th Congress of the United States of America. (2018). First step act of 2018 - public law 115–391. Congress.Gov. Retrieved November 28, 2021, from https://www.congress.gov/115/plaws/publ391/PLAW-115publ391.pdf

AAP, AAPOS, AACO and AAO Hoskins Center for Quality Eye Care. (2014, July 1). Joint Statement: Learning Disabilities, Dyslexia, and Vision - Reaffirmed 2014. American Academy of Ophthalmology. https://www.aao.org/clinical-statement/joint-statement-learning-disabilities-dyslexia-vis

Adams, M. J. (1990). Beginning to Read: Thinking and Learning about Print. Cambridge, MA: The MIT Press.

American Academy of Pediatrics, American Academy of Ophthalmology, American Association for Pediatric Ophthalmology and Strabismus, and American Association of Certified Orthoptists Joint Statement. (2009). Vision Problems Do Not Cause Dyslexia. Pediatrics, 124(2)

American Psychological Association (2006). See Brain. See Brain Read... http://www.psychologymatters.org/brainread.html. Copyright © 2006 by the American Psychological Association. Reproduced with permission

Brady, S. (2019). A Call for Changes. Perspectives on Language and Literacy, Volume 45, No. 1, 15-21.

Catts, H. W. (2018). The Simple View of Reading: Advancements and False Impressions. Remedial and Special Education, 39(5), 317–323. https://doi.org/10.1177/0741932518767563

Catts et al. (2017). Viewing a phonological deficit within a multifactorial model of dyslexia. Reading and Writing: An Interdisciplinary Journal, 30(3), 613-629.

Common Core State Standards in English Language Arts, pp. 15-17, Appendix A, pp 17-22.

Dehaene, S. Reading in the Brain: The New Science of How We Read (New York: Penguin Books, 2009)

Dehaene, S. lecture videos: Reading the Brain, April 7, 2012; How the Brain Learns to Read, World Innovation Summit for Education, 2012.

Dickman, E. (2017). Do We Need a New Definition of Dyslexia? The Examiner, 6(2).

Dorta, N. (2021, February 5). Is dyslexia genetic? Understood. https://www.understood.org/en/learning-thinking-differences/child-learning-disabilities/dyslexia/is-dyslexia-genetic

Ehri, L. C. (1984). How orthography alters spoken language competencies in children learning to read and spell. In R. Valtin & J. Downing (Eds.), Language awareness and learning to read (Springer Series in Language and Communication, Vol. 17, pp. 119–147). Springer.

Elliott, J. G., & Grigorenko, E. L. (2014). Cambridge studies in cognitive and perceptual development. The dyslexia debate. Cambridge University Press. https://doi.org/10.1017/CBO9781139017824

Farrell, L., Davidson, M, Hunter, M., and Osenga, T. (2010). The Simple View of Reading. The Center for Development and Learning https://www.cdl.org/articles/the-simple-view-of-reading/

Foorman, B., Beyler, N., Borradaile, K., Coyne, M., Denton, C. A., Dimino, J., Furgeson, J., Hayes, L., Henke, J., Justice, L., Keating, B., Lewis, W., Sattar, S., Streke, A., Wagner, R., & Wissel, S. (2016). Foundational skills to support reading for understanding in kindergarten through 3rd grade (NCEE 2016-4008). Washington, DC: National Center for Education Evaluation and Regional Assistance (NCEE), Institute of Education Sciences, U.S. Department of Education. Retrieved from the NCEE website: http://whatworks.ed.gov.

Friedman, M. (2018, October 23). IDA/LDA Whitepaper. International Dyslexia Association. https://dyslexiaida.org/ida-lda-whitepaper/

Friedman, M. (2021, March 23). Dyslexia Basics. International Dyslexia Association. https://dyslexiaida.org/dyslexia-basics-2/

Fuchs et al. (2008). Making “secondary intervention” work in a three-tier responsiveness-to-intervention model. Reading and Writing: An Interdisciplinary Journal, 21, 413-436.

Gough, P. and Tunmer, W. (1986). Decoding, reading, and reading disability. Remedial and Special Education, 7, 6–10.

Griffith, P., & Olson, M. W. (1992). Phonemic awareness helps beginning readers break the code. The Reading Teacher, 45.

Hanford, E. What Teachers Should Know About the Science of Reading. https://www.facebook.com/edweek/videos/2162504170510002/

Hennessey, N. The multifaceted nature of reading acquisition: Unraveling the Reading Rope. [Recorded webinar]. AIM Institute (2019). https://www.aimpa.org/institute/access2experts/multifaceted-nature-of-reading-acquisition-waitlist-recording-request/multifaceted-nature-of-reading-acquisition-recording

IDA Editorial Contributors. (2018, July 16). Definition of dyslexia. International Dyslexia Association. https://dyslexiaida.org/definition-of-dyslexia/

Moats, L. (1999). Teaching reading IS rocket science: What expert teachers of reading should know and be able to do. https://www.aft.org/sites/default/files/reading_rocketscience_2004.pdf

National Institute of Neurological Disorders and Stroke. (2019, March 27). Dyslexia information page | National Institute of Neurological Disorders and Stroke. Dyslexia Information Page. https://www.ninds.nih.gov/Disorders/All-Disorders/Dyslexia-Information-Page

Odegard, T., ed. Dyslexia defined: An update with implications for practice. Perspectives on language and literacy, Volume 45, No. 1, Winter 2019

Office of Special Education and Rehabilitative Services (2015). Guidance to educators. https://www2.ed.gov/policy/speced/guid/idea/memosdcltrs/guidance-on-dyslexia-10-2015.pdf

Pennington et al. (2012) Individual prediction of dyslexia by single vs. multiple deficit models. Journal of Abnormal Psychology, 121 (1), 212-224.

Ring and Black (2018). The multiple deficit model of dyslexia: What does it mean for identification and intervention. Annals of Dyslexia, 68(2), 104-125.

Scarborough, H. S. (2001). Connecting early language and literacy to later reading (dis)abilities: Evidence, theory, and practice. In S. Neuman & D. Dickinson (Eds.), Handbook for research in early literacy (pp. 97–110). New York, NY: Guilford Press.

Scarborough, H.S. (1998). Early identification of children at risk for reading disabilities. In B.K. Shapiro et al. (Eds.) Specific reading disability: A view of the spectrum (pp. 75-119). Timonium, MD: York Press.

Seidenberg, M. (2017). Language at the speed of sight: How we read, why so many can’t, and what can be done about it. Basic Books.

Shaywitz, B. A. et al. (2004). Development of left occipitotemporal systems for skilled reading in children after a phonologically-based intervention. Biological psychiatry, 55 (9). 926-33. https://doi.org/10.1016/j.biopsych.2003.12.019

Shaywitz, S. (2003). Overcoming dyslexia: A new and complete science-based program for reading problems at any level. Alfred A. Knopf, p. 58., 78-81, p. 83.

Stanovich, K.E. (1986). Cognitive processes and the reading problems of learning disabled children: Evaluating the assumption of specificity. In J. Torgesen & B. Wong (Eds.), Psychological and educational perspectives on learning disabilities (pp. 87-131). New York: Academic Press.

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Figure 1. Catts, H. W. (2018). Decoding and language comprehension [Infographic]. Adapted from The Simple View of Reading: Advancements and false impressions. Remedial and special education, 39(5), 317–323.

Figure 2. Shaywitz, S. (2003). Sea of strengths model of dyslexia [Infographic]. From Overcoming dyslexia: A new and complete science-based program for reading problems at any level. Alfred A. Knopf, p. 83.

Figure 3. The Yale Center for Dyslexia & Creativity. (2017). Neural signature for dyslexia: Disruption of posterior reading systems [Image]. From Comments by Drs. Sally & Bennett Shaywitz on dyslexia & ADAAA. Retrieved January 7, 2022, from https://dyslexia.yale.edu/comments-by-drs-sally-bennett-shaywtiz-on-dyslexia-adaaa/