Sua,
It’s my understanding (keep in mind that I’m only in my first year of linguistic study) that region has a lot to do with it. In the past, accents have been self-propegating- people tended to stay in the general area where they were born, and their children picked up their own speech habits.
The biggest problem I see, honestly, is the word “accent.” You’ve met me, and I don’t think that you’d say I have a Delaware accent. Part of this is because I’ve had speech therapy, part of it is because I’ve worked to get rid of it.
However, when I’m at home, I speak with a stronger DE accent than when I’m not. You’ll find this is true with most any group. When conversting with someone from your own region, especially if one is not currently in that region, an accent will strengthen. It’s a subconscious sign of solidarity and identity.
The way we speak is entirely depending on both the speaker and the addressee. I speak differently to my parents than I do my brother than I do my teacher and so on. In reality, we all have a wide variety of “registers” that we use every day.
It’s been suggested that cultivation of accents can have a lot to do with creating an identity. In New Guinea, the place in the world for diverse and original languages, there are several languages which are in all respects virtually identical… but the speakers swear that they are different languages, and more also, that they can tell the difference between them.
Accents vary by region, town, socioeconomic group, and even gender. I disagree with your assertation of an “accentless” Mid-West. A person from northern Kansas would more than likely be able to tell someone from southern Kansas. I live in a truely puny state, but I can tell what part of it someone is from by hearing them speak. It’s just one of those things that native speakers in a region know.
Anyway, one of the things in the development of accents is that there’s a “meeting” of sorts. If, in an area, there is a distinct minority group who speaks a different language than those in power, there is a tendancy to adapt a system that is a combination of both languages… with a heavy influence on the language that is the more prestigious, i.e., spoken by those in power.
Ellis, your assertation is correct. Babies are born with the ability to use any phonetic system, but very quickly learn the one of whatever language they are addressed in.
I hope this helps.