Her voice sounded like money . . .

Chris Bertram asks:

On a friend’s recommendation, I watched the excellent Now, Voyager the other night. A very fine performance from Bette Davis, who makes the transition from dumpy and downtrodden to shining society beauty brilliantly. But enough of the plot spoilers. Especially in the opening scenes, everyone sounds upper-class English. Perhaps not as cut-glass as Brief Encounter , but close. Maybe some of the characters are supposed to be English (Dr Jacquith, played by the English Claude Rains might be), but others, such as the matriarch Mrs Henry Windle Vale (played by the English Gladys Cooper) are definitely supposed to be American (upper-class Bostonian). And Bette Davis herself, is, obviously, an American actor playing an American character (but still sounding English). So, did Bostonian aristocrats in the 1940s actually speak with English accents? Or were the dramatic conventions such that English actors (Rains, Cooper) didn’t have to change their voices?

It's odd that an entire American accent disappeared virtually overnight: the upper class American accent that covered not only the northeastern seaboard, but California as well. Some of my friends parents had it, and a few famous people are still hanging on, like former New Jersey governor Tom Kean. But the accent of the Roosevelts, Julia Child and Katherine Hepburn pretty much up and vanished sometime in the late 1950s.

Why did this happen? Television tends to flatten regional accents, of course, but how come Britain held onto its aristocratic tones, while America's slipped softly and silently away?