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2.1 The Definition of Dyslexia

The Colorado Department of Education has elected to use the following definition of dyslexia, established by the International Dyslexia Association (2002) and adopted by the National Institute of Child Health and Human Development (Lyon, Shaywitz and Shaywitz, 2003):

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.

Moats and Dakin offer the following explanation of the key pieces of this definition in Basic Facts About Dyslexia & Other Reading Problems (2008):

  • Specific learning disability is an impairment of learning ability that may affect one or more academic areas, but not others, and exists in spite of adequate intelligence and opportunity to learn. For example, a person may be good at math or mechanical problem-solving but poor at reading. Specific learning disability is also defined in special education laws and policies, often in different ways by different states. (See Chapter 8 for Colorado’s definition of Specific Learning Disability.)
  • Neurological in origin means that the person’s reading, language or writing problems arose from factors within that individual that have a basis in “wired-in” aptitudes for language learning and reading. However, the person’s environment and experiences in life also determine how well he or she learns.
  • Accurate and fluent word recognition is the person’s ability to read single printed words accurately and quickly and to read aloud with sufficient speed to support understanding.
  • Spelling and decoding abilities refers to the person’s ability to spell accurately and to read unknown words by using phonics or letter-sound correspondences and recognizing syllable patterns and other chunks of longer words.
  • A deficit in the phonological component of language is difficulty pronouncing, remembering or thinking about the individual speech sounds that make up words.
  • That is often unexpected means that in spite of typical classroom instruction, adequate intelligence and opportunity to learn, the person struggles with reading and/or writing more than other students at the same grade, age or ability level.
  • Secondary consequences means that students with dyslexia, because they cannot and do not read very much and are not “wired” to learn language easily, often have related problems learning the meanings of words and comprehending academic language as they progress through the grades.

(We gratefully thank the authors and the International Dyslexia Association for permission to use this detailed explanation.)

In this handbook, the term dyslexia will be used to represent a common but specific type of reading disorder also known as developmental dyslexia. Developmental dyslexia is a reading disorder that an individual is born with as a result of genetic, hereditary and/or neurobiological differences and subsequently develops over time. In contrast, acquired dyslexia is the loss of existing reading skills as a result of a traumatic brain injury such as head trauma or stroke. Developmental dyslexia, or dyslexia, is a lifelong condition regardless of the age at which it is identified or diagnosed. Simply put, dyslexia is a brain-based learning disability that specifically impairs a person's ability to read and/or spell.


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The National Center on Improving Literacy’s infographic What is Dyslexia? and the companion literacy brief, Defining Dyslexia, can be viewed at National Center on Improving, Defining Dyslexia web page.


Brain imaging studies have shown brain differences between people with and without dyslexia. These differences occur in areas of the brain involved with key language and reading skills. For individuals with dyslexia, language areas of the brain used for reading may not function in the same ways as they do in individuals without dyslexia.

Dyslexia is the most commonly diagnosed reading disorder. Dyslexia is also found on a continuum of severity, ranging from mild characteristics of dyslexia to profound difficulty with reading and writing. In its most severe forms, it is a learning disability. In its mildest form, it may be a source of puzzlement, frustration or mild inconvenience. As a result of this span of difficulty, the exact prevalence of dyslexia has yet to be definitively determined. It has been suggested that perhaps as many as 15% to 20% of the population as a whole have some of the symptoms of dyslexia (IDA, 2017). A recent white paper indicates that the prevalence has historically been reported as affecting 5% to 17% of children (Petscher, Fien, Stanley, Gearin, Gaab, Fletcher & Johnson, 2019). In a recent video interview with Dr. Nadine Gaab, the range of 8% to 12% was reported (Reading Rockets & NCIL, 2019).

The commonly accepted features of dyslexia include:

  • Difficulty with phonological processing, which impacts one’s ability to effectively decode letters into blended sounds to form words. A fundamental phonological processing problem may block access to more advanced aspects of reading, such as word identification and comprehension.
  • Slow, inaccurate or labored oral reading, i.e., lack of reading fluency.
  • Difficulty with spelling, as demonstrated in an inability to efficiently write the letters comprising words from memory; increased time needed to spell words; and spelling errors that may be apparent.
  • Difficulty with rapid naming may be evident, making it difficult to quickly retrieve the speech sounds and the correct letter-order patterns required to be an efficient reader or speller.

Common associated features include:

  • Difficulty acquiring and using oral and written language;
  • Difficulty learning and retaining multisyllabic vocabulary required for mastery of academic content; and
  • Limited reading comprehension due to weak decoding, word recognition and fluency skills.