What exactly is a dialect? In America, we don't have an official "high" version of our language the way Germans and Italians do, so how do we decide who speaks standard English and who doesn't?
Here in America, your speech is usually considered standard if it doesn't call attention to itself — if you don't use certain verb forms or certain kinds of plurals and so forth. As academics, we try to use "dialect" as a neutral term by saying that everyone speaks a dialect. But society doesn't quite see it that way. The word has taken on negative connotations.
A lot of politicians use dialect to their advantage.
Sure. We've looked at YouTube videos of President Obama in corner shops and so forth, and you can hear him doing just that. Or you can watch Bill Clinton's speech at the Democratic convention. When he goes off script, he starts using southernisms — dropping his "g"s and even using "liketa." That promotes a certain solidarity. It says, "I'm the same as you guys. I'm not above you."
As linguists, we distinguish between "covert prestige" and "overt prestige." For Obama and Clinton, shifting into dialect gives them a certain "covert prestige." It works for them because everyone knows they're very bright guys to begin with. So when they do it, it has a different function than it might for other people.
How far back does Southern American English go? Did Thomas Jefferson sound anything like Bill Clinton?
Actually, it was really after the Civil War that Southern speech started to develop. There was some of it present beforehand. But after the war, the North and South had a lot less to do with each other. That's when iconic phrases like "y'all" started to really take off. Even the vowel sounds became more embellished — saying "tahm" for "time" or "sayed" for "said." Those little vowel differences are the real indicators of Southern speech.
What about African American speech? Did it originally come out of the South?
There's been a temptation to trace it back to a single origin. But from the beginning, African American speech probably had a lot of variation. And over the last hundred years, it has become even more divergent from Southern speech. We have recordings of African Americans who were born around 1910 — when you listen to their speech samples, they sounded a lot more like the white people in their own regions than African Americans often do today.
What's the reason behind that change?
People in urban areas today are relatively segregated, and their lives tend to be wrapped up in their communities. That makes cities a fertile ground for different speech patterns to develop. There are certain characteristics that do seem to show up in the African American vernacular from Los Angeles to Boston. For example, there's the habitual "be" — as in, "my ears be itching" — or the dropping of the verb — as in, "she nice." But as white linguists, for a long time we got a little bit caught up in this exotic "other" and didn't realize how distinct some of the varieties actually are.