Why do people in different areas speak differently? Where do accents come from, how do they change and/or survive over time and why do we have them?
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What do we mean by "dialect"?
First of all, let me say that the distinction between "dialect", "slang", and "language" is fuzzy, arbitrary, and fundamentally a social (cultural, political) construct.
Two dialects of the same language can be mutually unintelligible (e.g. Moroccan and Baghdadi Arabic — significantly different on every level), while two separate languages can be almost 100% understandable (e.g. Serbian and Croatian — considered different for nationalistic reasons by some). You can also have a language continuum; for example, Dutch and German are completely different languages. If you start out in northern Germany, and start heading (along the correct path) from town to town, you'll find that each neighboring town understands each other, but once you are in the middle of the Netherlands, you'll realize that people are now speaking dialects of Dutch, and at some point you crossed over from one to another, but there was never a line where one group is speaking German and the other speaks Dutch.
Defining what a "language" is in real terms
A good way to think about why we have dialects and accents is if you don't think about a language as a single entity of some kind. Each official "language" is more of an abstract concept. Instead, you can think of language as a distributed system, where no member of the system (any speaker of the language) has complete knowledge of the system. Those members who come in contact with each other influence each other as they communicate and interact, meaning that there is bi-directional feedback between members. Due to things like innovation, misunderstandings, reanalysis, and borrowing, random elements enter into the system and drive change. Also, the population is always adding new members who are more malleable and losing older members.
The more isolated and coherent a group of people are, the more these types of variation lead to deviation in the form of the language, because the repeated interaction with members having similar grammar leads to a sort of feedback loop. (You can think of this as being somewhat analogous to isolated groups of people having genetic defects or features in common, because the repeated combining of similar DNA amplifies subtle mutations.)
This is happening all at the same time, with "group of people" being defined at all levels, and all updating and influencing each other at the same time. For example, each person has his own way of speaking, and households will often develop unique features in the way they speak. Different age groups, genders, and other culturally defined groups may have their own features as well. People's grammars are being influenced by their peers and, in turn, influencing their peers.
Looking at language in this way, it makes perfect sense that we would have all sorts of variation.
So, the real reason why we have dialects is actually because we arbitrarily choose one version of a language and deem that "the language", making every deviation from this official version a de facto dialect. In reality, we are a huge system of varying idiolects with varying levels of coherence.
Differences in language change today
There are certain things that drive a language towards greater and broader coherence nowadays. The biggest is education and a writing system. With everyone learning standard English in school, and a standard writing system, we maintain some connection to a certain version of a language that changes much more slowly and conservatively.
Another major factor that maintains widespread language coherence is media: books, radio, TV, Internet. On television, we hear a variety of different accents, and above all others, a "dialect-free" version of English. We can read books, webpages, and listen to the radio, and all of these things impact our own internal version of language.
For these reasons, language change in English might take a different form than it had for its entire existence until 100-200 years ago. It may be slower to change in certain ways, and faster in others, dialects may merge, or any number of other things.
With one or two artificial exceptions, languages are not made, they are grown. More, they are grown by the simple mechanism of usage. Languages are never set in stone, but are living things that change as people use them. The most simple and glaring example today is the 'Americanization' of, basically, all the languages in the world by the mechanism of mass media, internet, and high tech. The most common word in the world is 'OK', a piece of American slang from WW1 (the latest theory, Some say it's American pre-WW1). Today, it may be that every single person in the world who can speak a modern language knows what 'ok' means.
But 'television' is also widespread. 'Hamburger' is the same in most languages (given pronunciation difficulties, like in Japan) on and on. Thus, American English is being spread everywhere by the mechanism of simple contact. It's not a conspiracy, it's a function of the elastic nature of language and its easy ability to be influenced.
This is also true for accents. All isolated or semi-isolated groups of a certain size have accents (as well as dialects) due to a natural evolutionary process - ('mutations' are spread by usage). It starts with one person saying 'oo' instead of 'ow' for words that have an 'ou' in them, (e.g. out, about, flout, trout...) it spreads to two, then more, and before you know it, Canada is created ;-) . (For those who don't know, Canadian English and American English are all but identical except that Canadians say 'oo' and Americans say 'ow' when they speak a word with 'ou' in it.)
There are unifying factors as to language such as the printed media and there are diversifying factors. Distant groups of speakers, not in constant contact with the main group of speakers, quickly develop their own way of speaking. But the main factor of differences probably is that there are different ethic groups that have their individual habits of speech.
People need to communicate. Our human desire to communicate is so intense, we experience it as a need.
A language is a set of rules that helps people communicate.
A language has lexical rules established by dictionaries. It also has grammatical and syntactical rules. These rules are enforced in a very pragmatic fashion:
I rule the house! If you want to live with me, you must learn my language. Husbands and wives argue over the meaning of words, and the proper way to say things. Hopefully these arguments are respectful and mutually beneficial, but regardless, whoever wins the argument establishes the method of communication.
Children tend to hybridize any divergence in the linguistic patterns of their parents. I am a native speaker of English. My wife's mother tongue is a tribal language of Kenya, but she is fluent in Kenyan English. We both speak Swahili, but decided English would be the dominant language in our household. Our children have integrated subtle patterns of her Kenyan English with my native English to form a household micro-dialect.
We rule the market place! Families band together in viable commercial units. If you want to do business with us, you must use our trade language. Trade languages develop to maximize the benefit of trade between producers and consumers, and the traders tend to dominate the method of communication.
We rule our world! Commercial units band together in government jurisdictions. If you want the protection and favor of our government, you must use our official language. These government units form complex alliances and rivalries, and the most wealthy traders tend to dominate the method of communication. This coercive sociological communication strategy produces the truism:
Dominance in trade and government expresses itself in linguistic dominance. Italy, Spain and Portugal all vied for dominance, and so their dialects became languages even though they are all visibly the product of Latin. Eventually, the British Empire prevailed, saying to every region of the planet: "We want to trade with you! We have the soldiers, guns and bombs, so you will trade with us. To ensure the steady flow of goods and services, we will deploy our soldiers, guns and bombs through our system of government."
The sun never sets on the British Empire was an axiom with a corollary:
Because of unique local and regional cultures, English is a language with innumerable dialects:
American English, Kenyan English, Indian English, Australian English all abide substantially by the rules of English, so they are not separate languages. For the sake of trade and peaceful international coexistence, each of those new nations agreed to maintain English as a language, even after they gained political independence.
In all languages, unique cultural influences in far-flung regions cause the languages to diverge into regional dialects over time. Consider the divergence tabulated below:
I was born in South East Pennsylvania, but my Philadelphian accent was modified because my family moved to North Central Pennsylvania when I was 13. The kids at school tortured me because I pronounced water as wooder: "*Wooder, wooder, wooder, whatchya gonna drink? How 'bout some water!*" To escape their scorn, I intentionally trained myself to say water the way they did. I smile when I visit back home and hear them say wooder.
International influences play a role in regional dialects. Kenyan English has a deeper Arabic influence through Swahili, which is fundamentally a hybrid of Arabic and Bantu language stocks. Starting in the port city of Mombassa, the Arabic influence is greatest, but as you travel inland, the Bantu influence overtakes the Arabic influence in both Swahili and Kenyan English. Indian English has a deeper Hindi influence that increases as you move inland from Mumbai. Australian English has a deeper Aboriginal influence.
The United States has become a prodigious melting pot of languages. Mexican Spanish has a much larger impact in Texas than it does in Maine, where people might recognize taco or burrito, but would be baffled by chillaquilles. Conversely, a Texas ranch hand would tilt his head in amusement if a Mainer talked to him about the davenport in his livingroom, because that is a French Canadian influence on their language.
Phonetic transformations among isolated people groups change the sounds, accent, and spelling of languages over time:
That regional phonetic divergence set up a linguistic competition between gaol and jail, which both have the same definition. Jail eventually prevailed, and we use that word with that spelling today.
Some American English dialects pronounce it:
in the South it is pronounced more like:
but it is spelled jail regardless of the pronunciation. That phonetic transformation is what we recognize as a regional accent, which is an element in the formation of dialects.
Bilingual immigrants often transform language by imposing the vocabulary, grammar or syntax of their mother tongue onto the learned language of their new home. When the Amish Mennonites moved to South Central Pennsylvania they spoke German, but they had to learn enough English to get along with their neighbors, but they did not learn English well. Because their religion demanded separation, they purposely butchered their English to reinforce a cultural distinction. If an Amish man speaks proper English, he is despised in his community and they say: "He's gone English!" It is not uncommon to hear an Amish man saying:
A native speaker of English would recognize those sentences as poor form, but they are word-for-word translations of correct Pennsylvania Dutch sentences. In Pennsylvania we refer to this as Dutch English.
The modern broadcasting industry uses an occupational dialect that is designed to maximize communication across dialects. It becomes a practical unifying measure of all other dialects because of its cultural connection to the ruling class.
These divergent forces are competing with the the unifying forces of language to create subsets. Each regional dialect fuses the regional rules of language with the regional rules of society. Each divergent group of people speaks a dialect, but they still need to communicate with the larger group to survive and thrive. Groups that depart farther from the core of the language are declaring a deeper and stronger sense of autonomy, but the larger group insists on absorbing the divergent forces for cultural, commercial, and political advantages.
The unifying forces of language tend to limit and slow the fracture of languages, but the divergent forces tend to generate subsets of regional, social, class, and occupational dialects.
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