One of my graduate classes in applied statistics we had people from across the country: Philadelphia, New York, South Carolina, Texas, New Mexico, Denver, California, Wyoming, and Washington State (and across the four major ethnic/racial lines in the US).
We thought it was interesting that we seemed to lack an accent, as opposed to those with whom we grew up, supposing the cause to be that we most identified with more educated people and thus trained ourselves up in a more educated voice.
Except the white girl from South Carolina: She had an accent as thick as she was.