The origins of the American accent

The short answer is that the different American accents are partly due to settlers from different parts of England (yes, and the rest of the world). The British Isles has a huge range of very distinct, very strong accents (even dialects in some parts).

So you have a bit of London mixed in with a bit of East Anglia, mixed in with some West country, etc and scattered across the US.

But, these accents were exported several hundred years ago. So, the sounds on both sides of the ocean have diverged, and the accents are no longer a neat match.

That said, there is a remote fishing community somewhere on the North Eastern coast that has an accent that is understood to be very close to "Shakespeare's" English (that is, from around Stratford in the late sixteenth, early seventeenth century).

There is often an implicit assumption that daughter communities innovate and the mother community keeps the original speech. But in the 400 years since London and New York began to diverge in accent, both have undergone 400 years of evolution.

A mother community (such as the homeland, England) being large and its colonies being small (at first), you might expect different rates of propagation of change. Well, I don't know how language change arises, and I don't think any linguist has much idea, but rather than big communities being conservative because there are so many people who have to accept change for it to get established, it seems to me there may be an opposite effect: if a change is spontaneous like a genetic mutation, then the more people there are, the more mutations have a chance to arise and be taken up by a subcommunity. (The situation is similar to, but might not be the same as, this question in biology.) These days most of the accent differences seem to be in big cities.

Many of the differences between American and Southern British are because of innovations in Southern British. The sound r was lost (except before vowels) somewhere over 200 years ago in London. This change spread out and is now established all across England except the south-west and East Anglia, and is also true of the Southern Hemisphere countries colonized in the last 200 years. So American resembles Irish in being a rhotic accent (one having r everywhere) because they are both survivors of the original situation that 300 years ago prevailed everywhere in England.

However, American has innovated on exactly how it pronounces its r: instead of being a simple consonant as in Irish English, it is now a colouring of the preceding vowel: see my How to pronounce an English "R" for details.

Again, the American can't with the same vowel as can reflects the original English, and it's England that's innovated to the ah vowel. This can be approximately dated by noting that Australia etc also say kahnt; but similar changes in dance, plant happened in England after the settlement of Australia, where they have not been taken up: the can vowel in common between American and Australian dance is also what was said in London 200 years or so ago.

One mutation which is characteristic of American is the "breaking" of the vowel in can and its fronting effect on the consonant, so that it is like kyean. Now this might be a new mutation, or it might be present in some of the dialects and accents of Britain that gave rise to it; I haven't the materials by me to tell. Bear in mind also that immigration from Britain to America was continuous over many generations, and new influences and changes could keep arriving. Language change is not purely divergent like a tree: changes arising in one place often propagate across dialect boundaries. Historically, London changes gradually influence most of the rest of England, and beyond; and in the last century American has achieved a similar driving position.

This is a very quick survey, of such a level of generality that I can completely ignore the differences within American English (see Cletus the Foetus's write-up there for details); and the possible influence of other languages such as Yiddish.

The prestige Boston accent, which is non-rhotic like London, is said to be a 19th-century innovation in imitation of London. However, I think this is an urban myth, and there is also a non-rhotic lower-class New York accent, which is highly likely to be native, not imitating England. See the first few entries in http://itre.cis.upenn.edu/~myl/languagelog/archives/2003_11.html for more detail.

Considerable detail can be found in The Cambridge History of the English Language Volume 6: English in North America, a book I haven't seen, but there's a good thorough review of it at
http://cf.linguistlist.org/cfdocs/new-website/LL-WorkingDirs/pubs/reviews/get-review.cfm?SubID=4807

A very brief sketch of a few major American accents and the groups of immigrants that contributed to some of their their major characteristics:

• New England = East Anglian Puritans
• Midwest, (such as Chicago) = German immigrants
• Northern Midwest (Wisconsin and Minnesota) = German and Scandinavian
• Parts of New York City = Italian, Irish, and Eastern European immigrants
• Southern seaboard (such as Carolinas, Georgia) = West Country English (This was Shakespeare's English)
• Appalachia (notably Tennesee and Kentucky) = Scotch-Irish immigrants from Northern Ireland (This accent is what Americans hear in Country Music)
• Deep South = perhaps some influence from West African languages via slaves (although this is still hotly debated)
• California - Californian and other western accents are especially interesting, if difficult to analyze because the region has recently had a massive influx of transplants from all over the US. One theory is that the California accent tends to neutralize the more extreme or unusual features of other regional dialects, creating a sort of bland, unoffensive homogenous mix. This process is supported by Hollywood's disdain for anyone with a strange accent - most movie stars and TV personalities have to learn a certain type of neutral accent

Actually, the original dialect of American English is what is called by some a 'high Southern accent', characterized by Civil War movies in which the Southern generals from Virginia say things like "Miss Scaalet, you aa looking mahty luvly tuhdey...I do hope the guvuhnuh shows up" This is a bad transcription, but hopefully yall can catch my drift. This is the 'proper speech' often heard (less common nowadays) in the legislatures of Virginia and Georgia.

This accent, interestingly enough, comes from lower class accents in England at the time Virginia was colonized. What's funny is that this is probably the closest to Shakespearean English that anyone gets nowadays. If you pay attention, the accent of Liverpool, England, long considered 'low-class', sounds pretty phonetically similar to this 'high southern' accent. If you don't pay attention sometimes, Paul McCartney can sound like he's from Richmond!