Is Dyslexia Genetic? What Causes It

Yes, dyslexia is partly genetic, and it often runs in families. Twin studies link between 40 and 70 per cent of the differences in reading between children to their genes. There is no single dyslexia gene, though, and a family history raises a child’s chance of dyslexia without deciding it.

What a parent usually wants to know next is what that means for their other children, what is going on in the brain, and whether anything they did played a part. This guide answers each in turn. For what dyslexia is in the first place, start with what dyslexia is and how it begins in spoken language.

Is dyslexia genetic?

Dyslexia is strongly influenced by genes. Researchers study this by comparing identical twins, who share all their genes, with non-identical twins, who share about half. A review of two decades of dyslexia genetics research reports that most estimates of heritability for reading skills and dyslexia fall between 40 and 70 per cent.

Heritability describes a population, not one child. A figure of 60 per cent does not mean a child’s dyslexia is 60 per cent genetic. It means that, across a large group, about six in ten of the differences in reading ability between children are linked to differences in their genes.

The same review describes dyslexia as “highly polygenic”. Many genes are involved, and each adds only a very small effect. A 2022 study of more than 50,000 adults with dyslexia and more than a million without, found 42 places in the genome linked to dyslexia. None of them causes dyslexia alone, and there is no genetic test that tells a parent whether a child has, or will have, dyslexia.

Is dyslexia hereditary? What a family history means

A child with a parent, brother or sister who has dyslexia is much more likely to have it too. In a review of family studies published in Psychological Bulletin, an average of 45 per cent of children with a close relative with dyslexia went on to have dyslexia themselves. In the comparison groups of the same studies, children with no family history, the figure was about 12 per cent.

Read the other way, more than half of the children with a family history did not develop dyslexia. A family history is a reason to watch early and to act early, not a forecast.

healthdirect, the Australian Government’s health information service, puts it simply: people with dyslexia often have a family member with dyslexia or with other reading and spelling problems.

What happens in the brain in dyslexia?

Dyslexia is a difference in how the brain processes language, not damage and not a lack of effort. healthdirect describes it as a neurological condition and notes that studies show the brains of people with dyslexia work differently.

In plain words, reading relies on a network of language areas on the left side of the brain that link the sounds of speech to written letters. A review in The Lancet describes dyslexia as a difference in how that network works. Brain imaging shows less activity in two areas towards the back of the left side when people with dyslexia read.

These differences are about language, and they start before reading does. The same review reports that infants with a family history of dyslexia already respond differently to speech sounds, and that some brain differences appear before reading is taught. At the level of thinking skills, the main difficulty for most children is working with the separate sounds in spoken words. That is why dyslexia is so often heard in a child’s talking before it is seen in their reading, as the guide to dyslexia and spoken language explains.

Can the environment play a part?

Yes. Genes raise or lower the chance of dyslexia, and a child’s experience of language and teaching shapes how it shows. The Lancet review reports that the heritability of dyslexia is lower in families with less formal education, and suggests the language and early reading experiences children have at home as one likely reason.

Teaching also matters. A child who has not been taught to read clearly and systematically can struggle for that reason alone. A child at higher genetic risk who gets early, explicit teaching may have less difficulty than they otherwise would. This is one reason a family history is worth acting on.

What does not cause dyslexia?

Several worries come up in almost every family, and none of them is a cause.

Eyesight. healthdirect states that dyslexia is not caused by problems with eyesight. A child should still have an eye test if reading is hard, because poor vision makes everything harder, but glasses will not fix dyslexia.
Intelligence. healthdirect states that dyslexia is not due to low intelligence, and a review in the Oxford Review of Education notes that it occurs across the whole range of ability.
Laziness or not trying. Dyslexia is a difference in how the brain handles language. It is not a matter of effort.
Parenting, screens and diet. None of the research reviews this guide draws on lists parenting style, screen time or diet as a cause.

If guilt is part of the question, the honest answer is that dyslexia is strongly influenced by genes, and none of the research this guide draws on points to anything a parent did as the cause.

What does a family history mean for younger brothers and sisters?

A younger child in a family with dyslexia has a higher chance of dyslexia, and the early clues are in their talking. The Psychological Bulletin review found that children with a family history were, as a group, slower to develop language as babies and toddlers. In one study the gap remained at three and a half years, while late talkers without a family history tended to catch up.

The same review found that, before school, children with a family history who later had dyslexia were weaker on average at:

vocabulary, the number of words they know and use
grammar, putting words together into sentences
remembering new sounds and new words long enough to repeat them
noticing and playing with the sounds in words, such as rhymes and first sounds
saying words clearly and accurately (the evidence here is weaker)

None of these on its own means a child will have dyslexia. Together, with a family history, they are worth raising with a speech pathologist before school starts rather than waiting to see how reading goes. The guide to early intervention for dyslexia covers why starting in the first years of school makes a difference.

Does knowing the cause change how dyslexia is treated?

No. Whether a child’s dyslexia runs in the family or appears without any family history, the teaching that helps is the same: explicit, structured teaching of how sounds and letters work, with plenty of practice. There is no treatment aimed at genes, and no test of genes that changes the plan.

What does change the plan is which skills are weak for this child: hearing and working with sounds, knowing letter patterns, reading words accurately and fluently, and spelling. Dr Lauren Crumlish, a Certified Practising Speech Pathologist, starts with an assessment of reading, spelling and the spoken language skills underneath, and builds the teaching from what it shows. A formal diagnosis of dyslexia is usually made by a psychologist. How the teaching runs from there is set out on the page about reading help for children with dyslexia.