There are several theories about the causes of dyslexia, but it is generally accepted to be a condition passed on through families.

It has been shown that if you have dyslexia, there is a significant chance your child will also have the condition, and if one identical twin is born with dyslexia, it is very likely the other twin will also have it.

Research has shown there are six possible genes that may contribute to dyslexia; however, there are thought to be many factors that cause the condition. Four of the genes have been shown to affect neuronal migration, which is part of the process in the brain's development that leads to specific areas of the brain having specialised functions.

This idea is also supported by research where brain scans have shown problems in the occipito-temporal cortex, which is an area towards the back of the brain.

It is thought these problems in the brain can contribute to dyslexia by affecting what is known as "phonological processing".

Phonological processing

The most widely supported theory of how dyslexia affects reading and writing is known as the "phonological processing impairment theory". To better understand this theory, it is useful to distinguish between how spoken and written language are understood.

The ability to understand spoken language seems to be a natural capacity of the human brain. This is why children as young as three years old can often speak and understand relatively complicated sentences.

As a result of this natural ability, when we listen to spoken language, we do not register that a word is made up of phonemes (the smallest units of sound that make up words). We only hear the word itself.

For example, when you hear the word "crocodile", you hear it as one word. You do not have to break up the word into its phonemes and then reassemble them to make sense of it (which would be the sounds "crok", "o", "dyle").

The same is not true of reading and writing. Both these skills require the ability to first recognise the letters in a word, then use these letters to identify the phonemes and assemble them to make sense of the word.

This ability is known as phonological processing. It is thought that people with dyslexia find phonological processing much more difficult than other people, because their brains function in a different way.