Academic Quarterly

Reading Expert Fall 2021

Toward a New Definition of Dyslexia

In 2003, the International Dyslexia Association (IDA) adopted a definition of dyslexia that has helped educators, legislators, and researchers in the US better understand what dyslexia is, what it is not, how to diagnose it, and how to provide the best possible instruction and support for students who have been diagnosed with dyslexia or show signs of word reading difficulty. The definition is as follows:

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.


(Lyon et al., 2003, p. 2)

Since 2003, more has been learned about dyslexia and some researchers are recommending an update to the definition. Brady (2019) suggested that an updated definition should consider how dyslexia symptoms manifest differently depending on the language’s opaqueness or transparency of orthography. English and French are considered to have more opaque orthographies. These languages have many phonemes that are spelled with multiple, varying letters (graphemes). For instance, in the English word eight, the eighgrapheme represents the long a sound. The long sound a can also be spelled a, a_e, ai, and ay in English. Thus, the orthography of English is considered “opaque” since the spelling system isn’t clear and consistent. In contrast, languages like Spanish and Finnish have a relatively consistent one-to-one orthography where a sound (phoneme) is spelled only one way rather than multiple ways. These orthographies are viewed as “transparent” and are much more clear and easier to learn.

Opaque and transparent orthographies usually cause more or less difficulty for students learning to decode words and achieving accuracy with decoding. Due to the ease of learning transparent orthographies, even students with dyslexia typically will learn to decode more easily and have fewer problems with accuracy than those who are learning to read an opaque orthography like that of English. However, those students with dyslexia learning a transparent orthography like that of Spanish will likely show difficulty with automaticity in reading words, ultimately impacting reading fluency. English-speaking students with dyslexia learning to read usually struggle with both learning to decode, decoding accuracy andautomatically recognizing words due to the lack of clarity (opaqueness) of the English spelling system. Brady (2019) recommends that a new definition should provide “summaries of characteristics for specific orthographies [that] are likely to be beneficial for practitioners and parents” (p. 20).

Brady (2019) also suggests that the narrowness of the IDA definition be addressed. This narrowness that Brady refers to describes dyslexia as a “deficit with the phonological component of language.” Although this phonological deficit is very often present in students with dyslexia, it has become evident over the last two decades that different cognitive profiles exist. Snowling, Hulme, and Nation (2020) report that “phonological difficulties are neither necessary nor sufficient to account for dyslexia” (p. 504). They cite evidence of children whose families have a history of dyslexia. These children have phonological awareness deficits but do not acquire reading difficulties. They also cite research that suggests that variations in semantic knowledge and oral language (Nation & Snowling, 1998; Hulme et al., 2015) are related to dyslexia. Other research has studied how paired associate learning or statistical learning are associated with dyslexia. Thus, while many students exhibit a phonological weakness, it is often not the only weakness observed. Furthermore, it is possible that phonological weaknesses can occur as a result of earlier language difficulties that can then cause difficulties beyond decoding into reading comprehension. This adds to the complexity of dyslexia.

Despite a wealth of research data showing the benefits of phonemic awareness intervention for students with dyslexia, it is now acknowledged that there is variability in cognitive profiles for students with dyslexia and that phonological deficits may not be the cause of dyslexia in all cases. This cognitive variability is helping researchers to understand that there may not be one single cause of dyslexia as once thought (Fletcher et al., 2019).

In 2009, the British Dyslexia Association (BDA) adopted the Rose Report’s definition of dyslexia:

Dyslexia is a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling.

Characteristic features of dyslexia are difficulties in phonological awareness, verbal memory and verbal processing speed. Dyslexia occurs across the range of intellectual abilities. It is best thought of as a continuum, not a distinct category, and there are no clear cut-off points. Co-occurring difficulties may be seen in aspects of language, motor co-ordination, mental calculation, concentration and personal organisation, but these are not, by themselves, markers of dyslexia. A good indication of the severity and persistence of dyslexic difficulties can be gained by examining how the individual responds or has responded to well-founded intervention. (pp. 9–10)

Although the International Dyslexia Association has yet to adopt a new definition for dyslexia, some are considering other examples such as this BDA definition to help think through what may need to be changed in the IDA definition. It is sensible here to point out that the BDA and IDA definitions differ in a few areas. One difference is that the BDA definition provides the more complete description of the characteristic features of dyslexia. Describing these features as “difficulties in phonological awareness, verbal memory, and verbal processing speed,” the definition recognizes a broader picture of varying cognitive profiles that students with dyslexia may exhibit.

The BDA definition also states that dyslexia is “best thought of as a continuum.” This also is an important feature of dyslexia that helps those using this definition to continue to research how interventions and supports can be crafted differently for those with mild, moderate, or severe cases of dyslexia.

The next sentence in the BDA definition addresses an important fact:  Difficulties in language, difficulties with attention and motor coordination, and math disability co-occur, often with the most severe cases of dyslexia. Snowling, Hulme, and Nation (2020) state, “None of these comorbidities should be viewed as ‘core’ features of dyslexia, but they can complicate both its presentation and response to intervention” (p. 506).

The last sentence of the BDA definition recognizes that the way to differentiate dyslexia from more “garden-variety” reading difficulties is to see how children respond to “well-founded” intervention. “Well-founded” interventions are those that are aligned to the evidence-base. If a child responds positively to intervention and the reading difficulties have been remediated, then it is not correct to diagnose the child with dyslexia. However, if the child does not respond to the intervention or responds less robustly, this then speaks to the persistence of the problem, which is a clear marker for dyslexia. Intervention approaches should address co-occurring difficulties that may require additional instructional intervention but also mental health interventions to address co-occurring depression or Attention Deficit Disorder that is commonly present among students with dyslexia.

Reading scientists continue to research dyslexia but there is still much to be learned. We know that a phonological deficit is quite often present in students with dyslexia and that interventions that address this deficit have proved successful for the vast majority of students. However, researchers have been questioning whether a phonological deficit is a causal factor for dyslexia or a symptom of it. We also know that different orthographies influence decoding accuracy and fluency in different ways. This knowledge is important to understand and continue to build upon. More research is needed to uncover this question. A new and updated definition can help move the needle toward more accurate ways to diagnose and intervene for dyslexia.


Brady, S. (2019). The 2003 IDA Definition of dyslexia: A call for changes. Perspectives on Language and Literacy, 45(1). International Dyslexia Association.

Fletcher, J. M., Lyon, G. R., Fuchs, L. S., & Barnes, M. A. (2019). Learning disabilities: From identification to intervention (2nd ed.). New York, NY: Guilford Press.

Hulme, C., Nash, H. M., Gooch, D., Lervåg, A., & Snowling, M. J. (2015). The foundations of literacy development in children at familial risk of dyslexia. Psychological Science, 26(12), 1877–1886.

Lyon, G. R., Shaywitz, S. E., & Shaywitz, B. A. (2003). A definition of dyslexia. Annals of Dyslexia, 53, 1–14.

Nation, K., & Snowling, M. J. (1998). Semantic processing and the development of word-recognition skills: Evidence from children with reading comprehension difficulties. Journal of Memory and Language, 39(1), 85–101.


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