Until the 15th century, Koreans wrote using Chinese characters. During that century, they invented an alphabet, Hangul, which a linguist of my acquaintance used to describe as the best alphabet ever created. Learning to read and write in Chinese characters took a very lengthy education. Learning to read Hangul, for someone who knew Korean, should have taken only a few days, long enough to memorize the sounds of the letters. So the introduction of Hangul should have converted Korea from a society where only the elite were literate to a society where almost everyone was.
China retained the traditional writing system. So the history of the two countries ought to provide information on the effect of widespread literacy. In what ways in which the societies were similar before Hangul did they diverge thereafter?
I don't have an answer, but it occurred to me that someone much more expert in the history of both countries might.