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Dyslexia is a complex, lifelong disorder involving difficulty in learning to read or interpret words, letters and other symbols. Dyslexia does not affect general intelligence, but is often co-diagnosed with ADHD. There are at least three sub-types of dyslexia that have been recognized by researchers: orthographic, or surface dyslexia, phonological dyslexia and mixed dyslexia where individuals exhibit symptoms of both orthographic and phonological dyslexia. Studies have shown that dyslexia is genetic and can be passed down through families, but it is important to note that, although a genetic disorder, there is no specific locus in the brain for reading and writing. The human brain does have language centers , but written language is a cultural artifact, and a very complex one requiring brain regions designed to recognize and interpret written symbols as representations of language in rapid synchronization. The complexity of the system and the lack of genetic predisposition for it is one possible explanation for the difficulty in acquiring and understanding written language.
Furthermore, recent evidence has found that there are certain genes responsible for causing dyslexia. Research also suggests a clear genetic basis for developmental dyslexia with abnormalities in certain language areas of the brain. However, there is also evidence that orthography, the correspondence between the language's phonemes and its graphemes , plays a significant role in the type and frequency of dyslexia's manifestations. Some psycholinguists believe that the complexity of a language's orthography affects the severity and occurrence of dyslexia, postulating that a more regular system would reduce the number of cases of dyslexia and/or the severity of symptoms.
Current psycholinguistic models of dyslexia are "largely developed on the basis of alphabetic writing systems such as English", but the amount of research on some logographic orthographies, Chinese in particular, is also fairly significant. Unfortunately, little research has been done on syllabic writing systems, and "cross-linguistic studies of the acquired dyslexia and dysgraphias are scarce."