The Wikipedia article linked to in my previous post (click on the heading) is very long, and when re-examining it a bit today I found that there is much more information to be found in it:
Scientific research has given rise to several theories of developmental dyslexia. They should not be viewed as competing, but viewed as theories trying to explain the underlying causes,
The phonological hypothesis
The rapid auditory processing theory
The visual theory
The cerebellar theory
The magnocellular theory
Perceptual visual-noise exclusion hypothesis
A note on the effect of language orthography is mentioned. Since we have three languages in use in our home when studying (and I'm a linguist myself), I find this interesting:
"Some studies have concluded that speakers of languages whose orthography has a strong correspondence between letter and sound (--) suffer less from effects of dyslexia than speakers of languages where the letter is less closely linked to the sound (--).
In one of these studies, --, the word-reading accuracy of first-grade children of different European languages was measured. English children had an accuracy of just 40%, whereas among children of most other European languages accuracy was about 95%, with French and Danish children somewhere in the middle at about 75%; Danish and French are known to have an irregular pronunciation.
However, this does not mean that dyslexia is caused by orthography: instead, Ziegler et al. claim that the dyslexia suffered by German or Italian dyslectics is of the same kind as the one suffered by the English ones, supporting the theory that the origin of dyslexia is biological. However, dyslexia has more pronounced effects on orthographically difficult languages."
A language with strong correspondence between letter and sound is called phonological. Because of languages being different in this regard, there can be differencies in reading ability depending on the language that you're dealing with. Arabic and Swedish are very phonological, but both have a few exceptions in their rules. English on the other hand is a night-mare for a lot of people! Though it does have rules (plenty of them) and exceptions to those rules, it is manageable, but it takes longer. That is why I'm reluctant to teach reading in English, actively, just yet.
Some links for more:
Children of the Code
Dyslexia, the Gift
Myomancy, about ADHD, Dyslexia and Autism "Find out what the Doctors and Teachers don't know about diagnosing and treating ADHD, Dyslexia and Autism"
Dyslexiaglasses by John Hayes, who left a comment on my previous post