No, @morphail, I’m not familiar with that study. If you’re an amateur in this field, I don’t know what I am. Beyond public school, I’m self-taught in the field, from literature I chose without guidance. A conventional English major such as mine was does not touch on any of this. My longed-for PhD in linguistics was not to be.
Maybe Labov’s finding is not controversial. That doesn’t make it a universal truth. In any case, I see that it’s about pronunciation, not grammar. I don’t believe they’re closely enough aligned that a study of one extends to the other. I would be arguing a different position if this thread were about pronunciation.
People use accent and diction to identify with a group—that much is obvious. Shibboleths. Who’s “us” and who’s “them”? I learned to speak the jargon of a computer programmer at one stage in my career. But @morphail, in the town and community of New England in which I grew up, where speech is (was) most decidedly a stamp that distinguishes the locals from the outsiders, people used to remark on my speech. Some asked me where I was from—in my home town. I can explain this, but that is way off the topic.
All my life, when I have just been speaking in my normal manner, without attempting to seem like anything or impress anyone, I’ve had people say to me, “Why don’t you just speak English?” How do you feel about the implication that clear, careful usage and grammatical speech aren’t English?
I mention this simply to illustrate my point that my choice of how I speak is not a matter of group identification.
I liked @JLeslie‘s analogy too, but I don’t think she was speaking about fashion. She was speaking of dressing appropriately for the occasion. I mentioned something similar when I said I don’t use my formal-banquet manners at the family dinnertable. Likewise, you can play your violin in a Mozart string quartet, or join the orchestra of a Broadway musical, or you can saw a hot fiddle in a country-music hoedown. Those are vastly different styles. You still want to play the right notes.