Dyslexia is rooted in how the brain develops and processes language, not in intelligence or effort. It affects roughly 20% of the population, making it the most common learning disability. The causes are a mix of genetics, brain structure differences that form before birth, and the way certain neural pathways handle the building blocks of reading.
Genetics Play a Major Role
Dyslexia runs strongly in families. Twin studies estimate its heritability at 44% to 75%, meaning genes account for a large share of who develops it. If one of your parents has dyslexia, your chances of having it rise significantly.
Researchers have identified nine regions of the genome linked to reading performance. The most consistently replicated genes sit on chromosome 6, in a region called DYX2. Two genes there, KIAA0319 and DCDC2, appear repeatedly across studies of different populations. These genes are involved in early brain development, specifically in how neurons migrate to their correct positions in the growing cortex. When these genes carry certain variants, that migration process can go slightly off course.
The Brain Develops Differently Before Birth
One of the leading explanations for dyslexia traces it back to the prenatal period, when the brain’s outer layer (the cortex) is still forming. Normally, newly created neurons travel outward from deeper brain structures to their designated positions in the cortex. In dyslexia, some of these neurons appear to end up in the wrong place, creating small clusters of mispositioned brain tissue called ectopias.
Post-mortem studies of dyslexic brains found between 30 and 140 of these tiny malformations per brain, concentrated around the left side in regions critical for language processing. These displaced cells don’t just sit in the wrong spot. They alter the local wiring, changing how nearby brain areas connect and communicate with each other. The result is subtle but meaningful: the neural architecture that supports reading is built on a slightly different blueprint.
Brain Structure and Wiring Differences
In most people, a language-processing area in the left temporal lobe called the planum temporale is noticeably larger on the left side than the right. In people with dyslexia, this leftward asymmetry is often absent. Brain imaging also reveals less gray matter (where processing happens) and less white matter (the cabling that connects regions) in left-hemisphere areas responsible for recognizing written words and linking them to sounds.
Functional brain scans taken while people read show a distinct pattern in dyslexia. A region in the left side of the brain that specializes in recognizing printed words, sometimes called the brain’s “letterbox,” shows weaker connectivity. Meanwhile, the right hemisphere picks up extra activity, as if compensating for what the left side isn’t delivering efficiently. There’s also persistent reliance on frontal brain regions associated with effortful speech processing, meaning reading requires more conscious work rather than becoming automatic.
The Core Problem: Processing Speech Sounds
The most widely supported explanation for what dyslexia actually disrupts is called the phonological deficit hypothesis. Reading requires your brain to map letters (graphemes) onto the sounds they represent (phonemes), then blend those sounds into words. This mapping process, called orthographic mapping, is what allows skilled readers to recognize words instantly without sounding them out.
In dyslexia, the brain struggles to process and store these sound units in short-term memory. The sounds themselves aren’t the issue; people with dyslexia can hear perfectly well. The problem is in how the brain breaks spoken language into its smallest pieces and holds those pieces steady long enough to match them to letters on a page. This is why children with dyslexia often have trouble with rhyming, sounding out unfamiliar words, and remembering sequences of verbal instructions, all tasks that depend on manipulating speech sounds mentally.
A Visual Processing Layer
For some people with dyslexia, difficulties with a specific visual pathway in the brain add another layer. Reading requires your eyes to land on each letter in sequence and track that order precisely. This sequencing depends on a set of large, fast-firing neurons called magnocellular cells, which detect timing and movement in the visual field. They don’t identify the fine details that distinguish one letter from another, but they direct your attention and eye focus to the right spot at the right moment so that other cells can do that detailed work.
When this magnocellular system is underdeveloped, several things go wrong at once. Visual fixation becomes less stable, so letters and words can appear to shift or blur on the page. The eyes struggle to converge precisely on text at reading distance. Attention moves more slowly and less accurately from one letter to the next. The result is that even when someone’s phonological processing is intact, the visual input arriving at the language centers of the brain is scrambled or out of order. Not everyone with dyslexia has magnocellular differences, but it explains why some people report that text seems to “swim” or move when they try to read.
Why It Often Shows Up Alongside Other Conditions
Dyslexia rarely travels alone. Between 12% and 24% of people with dyslexia also have ADHD, and the overlap runs in both directions: 20% to 40% of children with the inattentive type of ADHD have reading difficulties. Dyslexia and dyscalculia (difficulty with numbers and math reasoning) co-occur about 40% of the time. Writing difficulties overlap with reading problems at even higher rates, with a roughly 70% correlation between word reading and writing performance.
This clustering makes sense given that dyslexia stems from broad differences in brain development rather than a single broken circuit. The genes and neuronal migration patterns involved don’t affect reading in isolation. They shape how the brain organizes multiple systems for processing language, attention, and sequencing. When the underlying wiring is different, it often shows up across several related skills rather than just one.
What Dyslexia Is Not
Dyslexia is not caused by laziness, low intelligence, or poor parenting. It is not a vision problem that glasses can fix, though visual processing differences can be part of the picture. It is not something children outgrow, because the brain architecture underlying it was established before birth. The challenges may look different in adulthood as people develop workarounds and compensatory strategies, but the neurological differences persist.
Formally, dyslexia is classified as a specific learning disorder with impairment in reading. Diagnosis requires that reading difficulties have persisted for at least six months despite targeted help, that skills fall substantially below age expectations, and that the problems aren’t better explained by intellectual disability, uncorrected vision or hearing loss, or lack of adequate instruction. Some people aren’t identified until adulthood, when the demands of work or higher education finally exceed their compensatory abilities.