What causes dyslexia?

There are a great many possible reasons why children find it difficult to learn to read. For many, these difficulties can be explained by low opportunity, poor teaching and low ability. Other children however, may have been well taught and are perfectly bright in other ways, yet mysteriously they cannot learn to read. They are 'developmental dyslexics'.

Dyslexia runs strongly in families suggesting a genetic basis. Genetic studies suggest around 50% is inherited, prevalence across cultures is similar (and independent of socio-economic status and IQ), and more males than females are affected. Nine genes have so far been identified and several more suggested. The genes so far discovered all control the development of the brain. These cause differences in the brain structure which include abnormal collections of some nerve cells that fetch up in the wrong place during development which lead to miscroscopic differences in connectivity between neurones, and an unusual symmetry of the language areas. These abnormalities may particularly affect 'magnocellular'systems which are specialised for rapid information processing.

Dyslexics commonly have problems with visually identifying and ordering the letters in a word rapidly and accurately - 'an orthographic deficit' and also with rapidly and accurately identifying and ordering the sounds in a words - 'a phonological deficit'. Impaired development of magnocellular systems may cause problems with identifying and sequencing letters visually and sounds auditorily. The genes may also make magnocells more vulnerable in immune attack during development of the brain, and to nutritional deficiencies, e.g of the essential 'omega-3' fatty acids that are found in oily fish.