A、There is no obvious reason for the unbalanced distribution of dyslexia in the world B、The existing theories about dyslexia are

Dyslexia is a term used to describe a marked difficulty in learning to read despite normal intelligence and vision. The problem is universal, but research suggests it doesn’t affect every culture or language group equally. On China’s mainland and in Japan, for example, dyslexia rates are estimated at less than 5 percent compared to 10 percent to 20 percent in the U.S. There are intriguing theories as to why, and Japan has produced some important clues.
    Japanese children first learn to read and write in parallel phonetic alphabets, hiragana and katakana, each containing 46 characters relating to 46 different sounds. After conquering them, the student embarks on learning Chinese characters. According to Uno, who works for Japan’s National Center of Neurology and Psychiatry, just 1 percent of Japanese students have dyslexic problems in reading the phonetic alphabets, while 2 percent encounter problems with Chinese characters. The numbers jump a bit when it comes to writing—2 percent for hiragana, 3.8 percent for katakana and 5 percent for ideograms—but they’re still low by American standards.