When and why did British and American accents diverge?
By the mid-18th century, British commentators had noticed “barbarous” words (Francis Moore, 1735) and “the trace of corruption” (Samuel Johnson, 1756) in American English. And after the war for independence (1775–83), American patriots worried that US regional accents would be a barrier to national harmony. However, at this point, there wasn’t a clear distinction between American and British English, and this lack of national accent differentiation was a particular problem for American sailors. They looked and sounded British enough that Royal Navy press gangs could claim them as deserters, a contributing factor in the War of 1812.
Across Britain, English speakers had different accents, which might have sounded more or less like the accents in America. But in the late 18th century, a new kind of accent started to take hold among the upper-class and educated, later called Received Pronunciation (RP). RP sounds came to be thought of as “standard”, and the original, regional accents as “deviations” from that standard. Aspects of RP, like the “broad a” sound in car or bath, started to spread into the regions.
Meanwhile, Americans mostly weren’t taking on RP, despite its status in England. Migration and immigration meant that different English and “foreign” accents were mixing in America, and so American regional accents lost their edges, to the point where they seemed more similar to each other and less similar to British. These differences will have been noticeable in the first half of the 19th century, and they continue to diverge today.
M Lynne Murphy, professor of linguistics at the University of Sussex