Ralph W. Fasold, William Labov, Fay Boyd Vaughn-Cooke, Guy Bailey, Walt Wolfram, Arthur K. Spears, John Rickford, Are Black and White Vernaculars Diverging? Papers from the NWAVE XIV Panel Discussion, American Speech, Vol. 62, No. 1 (Spring, 1987), pp. 3-80
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Fasold et al. (1987) studied this question.