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American English is one variety of "World English." The conditions of its history and development are similar to those in Australia, New Zealand, Canada, Ireland, Nigeria, Kenya, South Africa, India, the West Indies, and other countries that were once part of the British Empire--always remembering that there are as many unique circumstances as there are different ex-colonies.
One major factor that distinguishes one variety of World English from another is the nature of the colonization. The United States resembles Canada, Australia and New Zealand in that the large indigenous populations in these areas were very quickly conquered, economically oppressed, and subject to European diseases that decimated them (in some places, like Tasmania, the native populations were completely eliminated). As a result, the people who speak English in these countries are largely descended from English immigrants and other immigrants who assimilated to the local variety of English. The relatively small native populations speak their own variety of English but have added little to Australian or American standards in the way of substrate.
That's not the case in the other places mentioned--Ireland, India, Kenya, Nigeria, South Africa are among the major English-speaking nations of the world, but in each case the dynamic is one of a few colonists imposing their language on a large number of native people. In most cases the colonists have themselves gone away, died out, or been assimilated or marginalized as a minority, but have left the English language as a dominant cultural legacy. In each of these situations, English encounters a powerful substrate of still-spoken languages: Irish, Hindi, Bengali, Gujarati and many other Indo-Aryan languages, Dravidian languages, Gikuyu, Ibo, Yoruba, Xhosa, Zulu, Afrikaans. These languages have dramatically inflected the vocabulary and phonology of these dialects of World English.
Native American languages have had a substrate impact on American English, of course. It is dramatic in the case ofplace-names, which tend to be from Indian languages. There is also a significant amount of American vocabulary drawn from Indian languages, some of which has spread outwards into British and World English.
American English and British English are remarkably similar in 2001--I say "remarkably" since the two countries have been politically independent of each other for well over 200 years and are separated by an ocean. Still, Americans are understood in England and English people are understood here. This fact should be compared to the divergence of English from continental dialects of West Germanic in the early Middle Ages, or to the profound changes in English between 1100 and 1300.
Phonological differences between American and British English are largely the result of American pronunciations staying conservative and British pronunciations tending to shift with time. That's counter-intuitive; Britain after all is the "old country" and America is the vibrant new innovator, right? But linguistically, a smaller and more isolated population will tend to conserve forms and usages. A larger, more "cosmopolitan" and metropolitan country will tend to change more quickly and noticeably.
An example from another language is the incidence of postvocalic /s/ in Spanish. In phrases like mas o menos or dos o tres, most Mexican Spanish speakers pronounce a final /s/ --therefore most people in Texas also have an /s/ at the end of those words. But Mexico (and areas of the US influenced by Mexican dialect) and Peru are the only two major Spanish-speaking countries where postvocalic /s/ is common.
Throughout the Caribbean, and in much of Central America and southern South America, the postvocalic /s/ has disappeared. The phrases I've given above sound like "mah o menoh," "doh o treh." Why? The loss of /s/ (we know that it was originally there because of the spelling) corresponds to a loss of that sound in southern Spain in the late 1600s and in the 1700s. The Caribbean countries kept in close contact with Spain and absorbed the change into their own dialect. (Argentina and Chile, settled later, brought the change over with them.) But Mexico and Peru, relatively isolated because of altitude and sheer distance from Spain, and relatively self-sufficient and politically powerful, ignored the change. So in one key respect, Mexican Spanish is like the Spanish spoken in Spain in the 1500s--as we might expect, given the history of the country.
The American analogy to Spanish /s/ is English postvocalic /r/. This sound is absent in southeastern England, especially in the RP; it's also absent in Australia, New Zealand, and much of the rest of the English-speaking world. It survives in Scotland, some parts of Ireland, and in most of the United States--though the /r/ is absent in New England, New York, and much of the Tidewater and Deep South.
What's going on here? This /r/ --as at the end of color and winter, more and star-- was pronounced in Old and Middle English, because we see it spelled out there. But between the first English settlement in America (mid-1600s) and the first settlement of Australia and New Zealand (after 1800), the sound disappeared (for unknown reasons) in the South of England. Southern English people transmitted their /r/-less accent to people in the port cities of the American colonies, to New England and to the tidewater South. But a substantial start on settlement of the interior of North America had been made by /r/ speakers who migrated westward through Pennsylvania and the Appalachians. That's where we find the /r/ today--in Appalachian speech, in the Midwest, and throughout the entire western US. Yet in the Deep South--southern Georgia, Alabama, Mississippi and Louisiana--the accent without /r/ spread because of settlement by /r/-less speakers from the Tidewater.
Let's look at the National Map of the Phonological Atlas of North America to get a sense of the major regions defined by the /r/ and many other features. Look here to see some of the documentation behind this map.