Dyslexia is a learning difference in reading that, like most learning differences, is defined in two different ways. Under the discrepancy model (traditionally used by private psychologists), dyslexia is a term used to describe a weakness in reading relative to an individual’s overall cognitive abilities. Many educational systems employ the absolute model of learning differences, which will consider students to have dyslexia if they consistently perform below grade level in reading regardless of their capabilities in other academic subjects.
It should be noted that while dyslexia is not defined in either of the primary classification systems used in psychology (the DSM-5 and ICD-11), dyslexia and a Specific Learning Disability (SLD) in reading, as defined by the DSM-5, are used by some as synonyms. The DSM stipulates that to qualify for an SLD in reading, a student must perform significantly below their grade level in reading for at least 6 months despite adequate instruction. Further, these difficulties must not be better explained by lack of proper instruction, other developmental disability, or other neurologic or sensory deficits.
Stating that a child is in the bottom 7% (or similar) of readers for their age group is not a dyslexia diagnosis, nor is it useful conceptually. Children can perform poorly in reading or any other subject for a multitude of reasons including overall cognitive delay, poor sensory processing, emotional distress, attentional issues, or inadequate instruction. The intervention that is most appropriate for a child falling behind due to having dyslexia will be very different than that for one with ADHD who can’t sit still in class. While identifying that a child is struggling with reading is an important first step, further assessment to determine the root cause of their struggle is crucial. For these reasons, while some individuals with an appropriate dyslexia diagnosis may meet the DSM criteria for an SLD in reading and vice versa, the gulf between these diagnoses is wide enough that the DSM should not be considered a primary tool in dyslexia diagnosis.
Given that it is most reasonable to define dyslexia as a discrepancy between reading ability and overall intellectual capacity, it must be noted that individuals with dyslexia may not fall significantly below their grade level in reading. Children with high general intelligence may have a disconnect between their overall ability and reading ability, but still be classified as an average reader in the absolute sense. These children will still be best able to reach their full academic potential if identified early and provided with appropriate interventions. Thus, it is important to remember that dyslexia can occur across a wide spectrum of intellectual abilities.
What is Dyslexia?
The estimated prevalence of dyslexia varies widely depending upon how it is defined, with estimates ranging from 3-7% all the way up to 20% of the population (Wagner RK, Zirps FA, Edwards AA, et al. 2020) This variation in and of itself is an indication that diagnostic clarity and accuracy must be improved. A key source of ambiguity is that reading ability exists on a continuum. If dyslexia is defined by the absolute model of learning differences (individuals performing poorly in a given subject regardless of their overall cognitive ability), then the cut-off point that defines under-performance is arbitrary.
Unlike learning to speak, which occurs organically through mimicking parents, most children need to learn to read through rote instruction. It is important to note that the most naturally able readers will learn to read without rote instruction but will do so at a diminished pace. However, most children will not reach their reading potential or learn to read to a functional proficiency without rote instruction. Evidence suggests that the vast majority of children learn to read in the same manner, regardless of whether they have dyslexia. However, people with dyslexia are far more likely to perform behind their reading potential, which is why a diagnosis, accommodations, and effective interventions are so important.
Over 40 years of research and meta-analyses (Hattie, J. 2009) demonstrate that Direct Instruction is the most effective reading program, with most other programs being ineffective. Direct Instruction emphasizes phonics instruction through carefully planned lessons designed around small learning increments. It is important to note that phonics is a necessary rather than sufficient ingredient of an effective reading program. A program that includes phonics instruction does not mean it is effective or evidence based.
Dyslexia is best defined as a language-based learning difference that results in difficulty with reading that is not expected in the context of the individual’s overall cognitive ability. While in the general population there is a correlation between overall intelligence and reading ability, this correlation is weaker in people with dyslexia. Of particular importance is that individuals with dyslexia struggle with reading and spelling but not with verbal and non-verbal reasoning.
While there is no single, universally accepted underlying cause, it is commonly believed that most dyslexic people struggle to read in large part due to poor phonological decoding abilities. Words in the English language are broken down into phonemes, which are small linguistic units that differentiate words from each other (e.g. ‘th’, ‘ng’, ‘er’, etc.). People with dyslexia struggle to automate the decoding of phenomes, an essential ability for both reading and spelling. As a result, people with dyslexia tend to read words as pictures (similarly to how people read East Asian languages). This leads to difficulty sounding out new words and occasionally interchanging words that look similar.
Diagnosing dyslexia early can prevent an educational snowball effect that can sabotage a child’s intellectual development. Because reading is so central to a child’s education, if they fall significantly behind in reading, they are subject to many adverse outcomes downstream that are unrelated to their innate intellectual capabilities. Their vocabulary and knowledge base may not expand as quickly as their classmates, and they are liable to fall behind in other subjects. Furthermore, the child may grow frustrated with school, become disruptive, and develop mental health issues.
Signs of dyslexia may be present before a child begins learning to read. During the preschool years dyslexic children may have difficulty recognizing rhyming patterns and struggle to learn the letters of the alphabet. In kindergarten and first grade, kids with dyslexia will often struggle to sound out even single syllable words and may struggle to associate letters with their corresponding sounds.
As dyslexic children progress in school they commonly try to avoid reading and activities that require reading. When forced to read they generally do so slowly and may be unable to sound out unfamiliar words such as proper nouns. They are also prone to make more spelling mistakes than their peers. Their lack of fluency and automaticity in reading may also cause them to run out of time on tests or fail to read instructions.
As adults, individuals with dyslexia may avoid reading for pleasure. They may also seek out careers that don’t require significant reading regularly. If reading and spelling errors still occur, these can be misinterpreted as carelessness which can be detrimental in a professional environment.
Assessing reading is simultaneously straightforward and complicated. Many schools conduct testing at the most basic level to simply identify students who are performing below grade and/or age level in reading. This can be accomplished with administration of formal achievement tests (most commonly the WIAT-4, WJ IV ACH, GORT-5, and NDRT) that evaluate reading comprehension, rate, and fluency. This process can be completed in under an hour but does not include any inference about etiology.
If the evaluator is trying to determine whether the student has a learning difference that transcends educational inadequacies and will require ongoing accommodations in reading (i.e., dyslexia), testing is more extensive. Formal achievement testing is still required, but fluid intelligence testing is also needed to determine whether a proper dyslexia diagnosis is warranted. The fluid intelligence testing is ideally conducted using the Wechsler testing batteries, which are what most dyslexia research is based upon and considered the gold standard. A thorough, targeted dyslexia test generally takes 2 to 3 hours.
Frequently, measures of phonetic processing and visual processing speed are also administered to help identify the student’s strengths and weaknesses, as the information obtained can help guide specific intervention strategies. Then achievement scores are compared not only to averages for the student’s age and grade, but also to their own predicted reading level given their overall intelligence in order to identify any significant discrepancy between the two. If the student is determined to be underachieving in reading given their intellectual potential, both evidence-based reading instruction and school accommodations are recommended.