Are Dyslexia and IQ Related?

Dyslexia is often diagnosed when a child or adult has more difficulty with reading than their IQ would suggest. New research adds to this discrepancy model by using fMRI patterns.


Although dyslexia is popularly thought of a tendency to reverse letters, it is more fundamentally an inability to match the sounds of letters with the symbols that stand for them. People with dyslexia have difficulty deciphering whole words, syllables, and individual letters. Young children with dyslexia may have difficulty learning the alphabet, rhyming words, counting syllables, or blending sounds to make words. Elementary school children may also struggle with slow and/or inaccurate reading, very poor spelling, and difficulty reading aloud.

The International Dyslexia Association has defined dyslexia as, "a neurologically based, often familial, disorder which interferes with the acquisition and processing of language." Dyslexia varies in severity, and may involve trouble with hearing the sounds of language, reading, writing, spelling, handwriting, and occasionally arithmetic. It is a lifelong impairment, but often responds to treatment, especially when help is offered early.

Educational accommodations and carefully targeted training during which the individual is taught the skills that come readily to a normally functioning reader: sound symbol correspondence, decoding, rhyming, the rules of spelling and phonics ... when these are all methodically taught and drilled using lessons that build one upon the other, many dyslexics can come to function quite well as readers.

Dyslexia is often diagnosed when a child or adult has more difficulty with reading, writing, and spelling skills than their IQ would suggest. This is called the "discrepancy model" because they are not performing up to the level that their thinking and reasoning abilities would predict.

Brain scans of dyslexics have frequently reported reduced activity in the left hemisphere in the neural circuits related to reading and language. While normal readers typically use specific parts of their brains when they read, dyslexic individuals do not. It is thought that this difference in neural processing causes inefficient and more difficult reading that leads to the problems that are encountered in deciphering and accurately connecting sounds and letters.

Recently, a group of researchers wondered whether the same neurobiological basis was at work in children who were poor readers and who also had lower than normal intelligence. Was it possible that the reading problems that the lower IQ children were experiencing were independent of their lower IQs and instead were related to brain circuitry abnormalities similar to those of normal IQ dyslexics?

The researchers used the same technology that has identified the functional brain abnormalities of diagnosed dyslexics. They compared the patterns of brain activity of poor readers with normal IQs and poor readers with low IQs to the patterns of typically developing children. They expected that the two groups of poor readers would show the same sorts of differences in brain-activation relative to typically-developing readers.

The functional magnetic resonance images (fMRI) taken of children with dyslexia with the low IQs had the same patterns of brain circuitry abnormalities as those of the children with dyslexia with normal IQs. Both groups had less activity in the two left hemisphere brain regions that are often less active in dyslexics.

This means that the children's reading disability is not related to their IQ alone but to identifiable patterns of abnormal brain function. The researchers concluded that should be looked for in all children with reading difficulties including those whose IQs are below average, rather than diagnosed only based on the discrepancy between a student's reading skills and her IQ.

They also propose that children with lower than average IQ should receive the same reading help that is offered to normal IQ children with dyslexia, as they too will benefit from the dyslexia-specific educational interventions.

Image: photo-oasis/Shutterstock.

This article originally appeared on, an Atlantic partner site.