[1] It stems from evidence that individuals with dyslexia tend to do poorly on tests which measure their ability to decode nonsense words using conventional phonetic rules, and that there is a high correlation between difficulties in connecting the sounds of language to letters (phonemic awareness) and reading delays or failure in children.
[2] The basic hypothesis is that reading failure or dyslexia stems from a functional or structural deficit in left hemispheric brain areas associated with processing the sounds of language.
Others have focused on the perception of short or rapidly varying sounds of language, positing that the core deficit is one of timing rather than of overall function.
[1] Since the 1990s, the phonological deficit hypothesis has been the dominant explanation favored by researchers as to the probable cause of dyslexia, but it is only one of several competing theories.
They also argue that much of the evidence for the theory is based on circular reasoning, in that phonological weakness is seen as both a defining symptom of dyslexia and as its underlying cause.