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What Causes Dyslexia? What the Latest Research Reveals

What Causes Dyslexia - index
Nine-year-old Peter is one of the brightest children in his 3rd-grade class. He has an amazing vocabulary and knows everything there is to know about soccer. But when it comes to reading about soccer, he has a lot of trouble.

It takes Peter a long time to read each word, and even longer to read whole sentences. He often has to guess at how you say a word — and sometimes his guess is wrong. Reading out loud is especially stressful and embarrassing. His teacher recently told Peter’s parents she thinks he might have dyslexia.

The term dyslexia was coined from the Greek words dys, meaning ill or difficult, and lexis, meaning word. It is used to refer to persons for whom reading is simply beyond their reach. Spelling and writing, due to their close relationship with reading, are usually included.

There is a labyrinth of differing, opposing, and often contradictory theories about dyslexia; what it is, its causes, and its possible correction. The cerebellum, a brain structure traditionally considered to be involved in motor function, has been implicated in developmental dyslexia. New research, however, shows that the cerebellum is not engaged during reading in typical readers and does not differ in children who have dyslexia (Ashburn et al., 2019).

The visual magnocellular deficit theory suggests that the difficulties in the visual processing of dyslexia are caused by the dysfunction of the magnocellular system (Stein, 2018). Miller (2015) says the biggest cause of reading difficulty is the unsystematic and unscientific teaching of reading. As a result, children are let down twice by the educational system: first by not being properly taught and then pathologized with the “diagnosis” of dyslexia when they fail.

We discuss the most common proposed causes of dyslexia in more detail.


Table of contents:

Dyslexia cause no. 1: Genetic influences

A large-scale study of twins with dyslexia yielded a concordance rate of 68% in identical twins, as compared with 38% in non-identical twins.

There is a large body of research on children at risk due to family history of dyslexia, including seven longitudinal studies covering age ranges from preschool/kindergarten through 2nd, 4th, or 6th grade. The estimated prevalence rate for dyslexia in the general English-speaking population is between 5 and 17%. However, the rate of reading-related skill deficits (e.g., word reading, orthographic coding, phonological decoding and phoneme awareness) based on familial risk studies is between 35 and 40% (Molfese et al., 2008).

The relative contributions of genetic influences and shared family environment can be dissected in twin studies. It has been shown robustly that a diagnosis of dyslexia is significantly higher in identical twins, who have a virtually identical genetic makeup, than it is in non-identical twins who (like ordinary siblings) share about half of their genetic makeup. A large-scale study of twins with dyslexia yielded a concordance rate of 68% in identical twins, as compared with 38% in non-identical twins, indicating a substantial genetic component (DeFries & Alarcón, 1996).
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Dyslexia cause no. 2: Cognitive deficits

Although some causes of dyslexia have a genetic origin, and environmental factors play an important role, cognition mediates brain-behavior relationships and therefore offers a sufficient level of explanation for the development of principled interventions. We thus need to understand the cognitive difficulties that underpin reading failure, regardless of whether their origin is constitutional or environmental (Elliott & Grigorenko, 2014).

Dyslexia research has been dominated by the quest for a single cognitive deficit that is necessary and sufficient to cause all behavioral characteristics of the disorder. Until the 1950s, the belief was that dyslexia is attributable to visual processing problems, perhaps also including motor skill problems. Visual processing refers to the brain’s ability to make sense of what the eyes see; visual memory is often considered to be a subset of visual processing rather than a separate skill.

But in 1957 Noam Chomsky published his seminal book, Syntactic Structures, which transformed the study of language and with it, reading. Dyslexia became a linguistic, phonological problem, and any role for visual processing was abandoned. In an influential book, Dyslexia: Theory and Research, Vellutino (1979) argued that many of the apparent visual problems could actually be attributed to language difficulties — especially to deficient phonological awareness.

Phonological deficit theory

The phonological deficit theory became the most well-developed and supported of the theories of dyslexia. The U.S. researchers have united in adopting the phonological deficit hypothesis since the early 1980s, and this united front has led to the investment of more than $15 million annually by the US government, via the National Institute for Child Health and Human Development (NICHD) (Fawcett, 2001). This research program into the causes and remediation of reading disabilities continues until the present day.

Phonological awareness (PA) refers to an individual’s awareness of the phonological structure, or sound structure, of language. It is a listening skill that includes the ability to distinguish units of speech, such as rhymes, syllables in words, and individual phonemes in syllables. Phonemic awareness is a subset of PA that focuses on recognizing and manipulating phonemes, the smallest units of sound. The two most important phonemic awareness skills are segmenting and blending.

The ability to segment and blend phonemes is said to be critical for the development of reading skills, including decoding and fluency, and even that it predicts reading ability (Edwards & Taub, 2016). It is also claimed that PA training can prevent and correct reading difficulties (Kilpatrick, 2016, p. 13). Moustafa, however, points out that correlation does not establish causation. “In statistics, the word predicts means nothing more than that there is a high correlation between two phenomena” (Moustafa, 2001, p. 248).

Not all studies support phonological and phonemic awareness training (Pape-Neumann et al., 2015; Krashen, 1999a; Krashen 1999b). Blomert and Willems (2010) investigated children at familial risk for dyslexia in kindergarten and first grade. The familial risk was genuine; 40% developed reading deficits in first grade. However, they did not find any relationship between a PA or other phonological processing deficits in kindergarten and reading deficits in first grade.

Some findings indicate that phonemic awareness may develop as a consequence of exposure to reading and writing, while other support an intermediate view, “that phonological awareness and alphabetic literacy learning influence each other reciprocally” (Manolitsis & Tafa, 2011, p. 31). Some researchers claim that phonological factors may be less important than is commonly accepted. Not all children with reading disabilities demonstrate a phonological deficit, and Catts and Adlof (2011) point out that children with poor phonological abilities can nevertheless develop good reading skills. Also, a single cognitive deficit model cannot account for comorbidity. Dyslexia co-occurs more often than would be expected by chance with other developmental disorders, such as ADHD and specific language impairment.

Multiple cognitive deficits

Given that a single phonological deficit is not necessary or sufficient to cause dyslexia, current thinking sees this as one of multiple cognitive deficits that are likely to interact to cause dyslexia (Pennington, 2006; Peterson & Pennington, 2012).

In addition to phonological awareness, cognitive psychology has now linked many brain-based skills to dyslexia:

  • verbal fluency;
  • attention and executive functions;
  • visual attention;
  • visuospatial abilities;
  • processing speed;
  • short-term memory;
  • auditory working memory;
  • visual and visual sequential memory;
  • visual long-term memory, especially for details;
  • verbal long-term memory; and
  • rapid naming.

Dyslexia linked to verbal and nonverbal IQ

Researchers have also found a link between dyslexia and verbal and nonverbal IQ. Van Bergen et al. (2014) assessed four-year-olds (N = 212) with and without familial risk for dyslexia on ten IQ subtests. Reading and arithmetic skills were measured four years later, at the end of Grade 2. Relative to the controls, the at-risk group without dyslexia had subtle impairments only in the verbal domain, while the at-risk group with dyslexia lagged across IQ tasks. Nonverbal IQ was associated with both reading and arithmetic, whereas verbal IQ was uniquely related to later reading. The children who went on to develop dyslexia performed relatively poorly in both verbal and nonverbal abilities at age four, which lends credence to the multiple cognitive deficit model.

Why cognitive skills matter

Weak cognitive skills prevent a process called orthographic mapping. Every word has three forms: its sounds, spelling, and meaning. The process of orthographic mapping involves the brain linking the three forms of the word and storing them together in long-term memory. Orthographic mapping allows for instant word recognition, fluent reading, and accurate spelling.
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Dyslexia cause no. 3: Brain differences

There is old but popular evidence consistent with the idea that there is something different in the brain of some persons who have persistent unexpected difficulty in learning to read. Specifically, Galaburda and colleagues (1985) reported several anatomical anomalies in the brains of a few persons with dyslexia. Although those findings are intriguing, they are far from decisive. The samples were extremely small (a total of four men and three women) and included participants with evidence for neurological or psychiatric conditions and participants with impairments not limited to written language, who should have been excluded from a study purporting to examine brain correlates of dyslexia. The control group was also very small, effectively precluding reliable estimation of the specificity of any findings.

Visual word form area

As technology advanced, neuroscience contributed more and more to dyslexia research. Unfortunately, most studies have too small samples to permit reliable conclusions to be drawn, and many results are inconsistent (Protopapas & Parrila, 2018). In a meta-analysis of functional neuroimaging studies of dyslexia, Martin et al. (2016) list studies in which differences between groups with and without dyslexia were found in specific brain regions. The most consistent findings concerned the left occipitotemporal cortex, which includes the so-called visual word form area (VWFA), though to be critical for reading.

Neuroscientists at Georgetown University Medical Center concluded that skilled readers can recognize words at a lightning-fast speed when they read because the words have been placed in a sort of visual dictionary. This part of the brain, the VWFA (shown in yellow), functions separately from an area that processes the sounds of written words (Glezer et al., 2016).

Data using fMRI indicate three neural systems for reading (shown in the image above). These are all located in the left side of the brain: one in the front of the brain (shown in green, in the region of the inferior frontal gyrus [Broca’s area]) and two in the back of the brain (one in the parietotemporal region, shown in red, and a second in the occipito-temporal region, shown in yellow). The latter system is of particular importance for skilled, fluent reading and is termed the visual word-form area (Shaywitz, 2005).

In the largest study of its kind up to date, Brem et al. (2020) confirmed that the VWFA is key to fluent reading.

Left parietal lobe

The left inferior parietal lobule came in a close second in the meta-analysis study by Martin and colleagues (shown in red). This part of the brain is said to be involved in word analysis, grapheme-to-phoneme conversion, and general phonological and semantic processing.

Imaging also reveals compensatory overactivation in other parts of the reading system (shown in green). The compensatory neural systems allow a dyslexic person to read more accurately. However, the critical visual word-form area remains disrupted and difficulties with rapid, fluent, automatic reading persist. The dyslexic continues to read slowly.

Brain differences do not equal brain disorders

Protopapas and Parrila (2018) point out that brain differences do not equal brain disorders, and that anatomical differences may be a consequence of reading experience as opposed to a cause of dyslexia (Krafnick et al., 2014).

It should also be noted that the brain is plastic. New connections can form and the internal structure of the existing synapses can change. New neurons, also called nerve cells, are constantly being born, particularly in the learning and memory centers. Approximately 700 new neurons are daily being formed in the brain. Neurons die each day too, keeping the overall number more or less balanced, with a slow loss of cells as we age. A person who becomes an expert in a specific domain will have growth in the areas of the brain that are involved with their particular skill.

The human brain is a powerhouse; the human brain has put a man on the moon, created the silicon chip that can do billions of calculations per second, invented red, yellow, and green lights to control millions of people in traffic every day and — believe it or not — found ways to see what goes on inside itself. The human brain itself tells us that it is most certainly capable of overcoming learning obstacles like dyslexia, despite genetic influences and brain differences.
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Tutoring for children with dyslexia

Edublox specializes in educational interventions that make children smarter, help them learn and read faster, and do math with ease. Our programs enable learners to overcome reading difficulties and other learning obstacles, assisting them to become life-long learners and empowering them to realize their highest educational goals.

Watch our playlist of customer reviews and experience how Edublox training and tutoring help overcome dyslexia signs and symptoms. Learn more about our approach to dyslexia treatment and book a free consultation to discuss your child’s learning needs. We cater to a variety of dyslexia types.


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Key takeaways

What Causes Dyslexia
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Authored by Susan du Plessis (B.A. Hons Psychology; B.D.), an educational specialist with 30+ years’ experience in the field of learning disabilities.
Medically reviewed by Dr. Zelda Strydom (MBChB).


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