"Revising the Language Map of Korea," by Changyong Yang, William O'Grady, & Sejung Yang; to appear in the book The Changing World Language Map, edited by Stanley D. Brunn, to be published by Springer.    

Summary: For much of modern history, the distinction between language and dialect has been largely political: the speech of a bigger or more powerful community is a language, whereas the speech of a smaller or less influential group is a dialect. In contrast, modern linguistics insists on a more principled distinction: at the point where two varieties of speech are no longer mutually comprehensible, they are classified as separate languages. Although Jejueo has long been classified as a non-standard dialect of Korean, we present evidence from an intelligibility experiment to show that it is not comprehensible to monolingual speakers of Korean and therefore should be treated as a separate language, in accordance with the usual practice within linguistics. 

    The complete paper can be downloaded below. If you'd like to see the intelligibility test for yourself or even take it, you can find at the page entitled "Jejueo Intelligibility Test 1."