To be a black American is usually to subconsciously acquire certain positionings of the tongue and lips, slightly different from the ones whites use, when producing certain sounds. Because in our times even that statement lends itself to misinterpretation, note the following: To be a white American is usually to subconsciously acquire certain positionings of the tongue and lips, slightly different from the ones black people use. This is purely observational. I cannot stress more that there is no value judgment here. Different groups of people speaking the same language—whether geographically separate or not—differ in terms of their vowels all over the world. Black people shape their mouths to speak in a certain way because the positionings happen to be the ones that the people closest to them are using when they are learning the subtle and complex thing that language is.
– John McWhorter, Talking Back, Talking Black: Truths about America’s Lingua Franca(2016)