In American English, are "mightn't have" and "might not have" both often used in speaking and writing?
How about "couldn't have" and "could not have"?
How about in British English? Thanks!
My gut feel is that the mightn't contraction is more common in BrE than AmE (but I've no reason to think most Americans find it at all "unusual").
I'm not sure if this is a valid way of using Google Books estimated results, but note these figures...
According to #1, there are three times as many instances of US color as there are UK colour. Since there's no reason to suppose Americans use the word any more often than Brits, I assume the relevant US corpus is about three times bigger than the US one.
All things being equal, we could expect the #2 AmE count to be about 18M, not 12M. But note that overall prevalance of might has fallen by a third in the last couple of centuries. Older texts in the Google Books corpus are more likely to be BrE, so the AmE aversion to might is weaker than the bare figures suggest.
But after allowing for the implication of the above (Americans are perhaps 20-30% less likely than Brits to use the word might in any given utterance), we should still expect the #3 AmE count to be 11K, not 4K.
Regardless of whether the absolute values of those Google Books estimates are accurate, I see no reason to suppose that the ratios aren't meaningful. So I think this validates my gut feel.
In American English mightn’t is extremely uncommon, but still understood.