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.
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