Is there any difference between じゃあない and じゃない?

Question

これっ！　石を投げるんじゃあないっ！
すいちゃったものはしかたがないじゃあないのっ！

I've been reading a light novel called Slayers, and I noticed something interesting: the author usually writes じゃあない and not じゃない. I'm curious if there are any differences between the two. (I think it's clear that they have the same basic meaning, so I'm mainly asking about differences other than meaning, like style/formality/dialect.)

Not long ago, I read a post on sci.lang.japan by the linguist Bart Mathias, which I'd like to quote here. He describes the contraction from ては to ちゃ:

Because Japanese is length sensitive, it took time to get the two moras of "-te-wa" compressed from "-chaa" to "-cha," and the earlier form still survives as an option.

My guess is that the same thing happened with じゃ:

では → じゃあ → じゃ

That is to say, I guess that じゃあ is older than じゃ. If that's right, I also guess that the combination じゃあない is older than じゃない, and since I usually hear the shorter form, I'm guessing that じゃあない might sound a bit old-fashioned. I'm curious if this is correct, so I decided to ask here.

Anyway, here are my guesses about じゃあない:

1. Perhaps じゃあない sounds like an older way of talking than じゃない.
2. Perhaps じゃあない sounds a bit closer to ではない (making it slightly less informal than じゃない?)
3. Perhaps じゃあない is a dialectal variation of じゃない.
4. Perhaps the author just likes the sound of じゃあない, and there's no real difference.

So tell me, is there any difference between the two? Or are they totally the same?

Answer 1

It would be interesting/useful to know if the author uses じゃあ over じゃ・では in other phrases.

I might be completely off-base here, but if we consider the difference as that between では、じゃ and じゃあ (rather than between じゃない・じゃあない specifically), there is some info about, for example here.

Taking some parts of the first answer on that question which I think are important (ignoring the bits about では for the moment):

First example:

これじゃ間に合わない　・・・普通
これじゃー間に合わない　・・・強調した言い方

e.g. in this case the じゃあ version is for emphasis.

Second example:

この雨じゃ出かけられない　・・・普通
この雨じゃー出かけられない　・・・リズムを変えた言い方（人によって好き好きです。）

e.g. in this case it just changes the rhythm (and this is a matter of personal preference).

Third example:

中学生じゃ無理だ　・・・普通
中学生じゃー無理だ　・・・強調した言い方。あるいはリズムだけを変えた言い方

e.g in this case it could be either for emphasis or a change in rhythm. (This may be the same way that sometimes 二 is read にい in counting out loud, e.g. いちにいさん). It appears from additional information given in the linked answer that the difference between the two cases would be distinguished in speech by the position of the accent.

Incidentally, 大辞林{だいじりん} confirms the origin from では but isn't much help in terms of nuance:

「では」の転。「じゃ」とも

Answer 2

If it's not for reasons of dialect, then I'm pretty sure it's for dramatic emphasis, as in to imitate the rhythm or someone drawing it out either as a kind of growl, or yell (the example sentences seem to be scoldings, to me), or just to create more dramatic effect.

I raise the idea of drawing it out for dramatic emphasis because じゃ　on its own is inconclusive - it could be followed by ある, instead of ない, and then the meaning would be reversed. So, by not immediately ending the phrase, there is an extra brief moment of dramatic uncertainty, even hope that you aren't about to be scolded -- before you finally, definitely are. :-)