The cost of a dropped copula*

You can file this one under the "obvious" category too, but it's interesting to see it verified academically. According to a paper by U. Chicago professor Jeffrey Grogger, a study controlled for socioeconomic factors found that "sounding black" will cost you about a tenth of the wage earned by someone who "sounds white," regardless of your actual race. Similarly, a southern accent will have about the same effect, though the penalty is a bit lower. (A note to non-native English speakers: though black English is similar to and has roots in southern English, it's very easy for Americans to differentiate between them.) I only read summaries of the paper, but a quick Cmd+F and the comments of a NYT blog post about the paper tell me that the study only looked at those two English accents/dialects, and didn't test the effects of having other sorts of regional accents (say, Midwestern or New York). Given that sounding black and sounding like you're from the South are about equally detrimental, I'd guess that the difference comes in people trying to use accent as a proxy for class, and associating black and southern English with being low class. The measure of racism is probably the differential between the hit you take from being from the South and the hit you take from being black.

* A linguistics joke – speakers of black English tend to drop the copula, which is any form of the verb "to be." See the AAVE Wikipedia article's section on grammar for more on the nitty-gritty linguistic details of black English.