I have long wondered about the phrase “aren’t I?” As contractions go, it runs afoul of the norm and this bothers me.
It was only recently that it bugged me enough to do some digging.
Logic would dictate that the proper phrase be “am I not?” But how would it be contracted?
“Are you not?” is really “are not you?”and is contracted as “aren’t you?” This makes sense.
But here’s the problem. “Are” does not agree with “I” in a sentence. “Am” does: I am. I am not. Am I not? Am not I? So then why not “amn’t I?”
Well, I consulted a lot of sources, and each took me deeper into obscurity.
This might not be the absolute truth, but what I gleaned from all I read is that “aren’t I” is incorrect but accepted. Just like plenty of words and phrases we’ve talked about here.
It also seems that, at one time, “amn’t I?” may actually have been considered correct in contemporary Scottish-English as an informal contraction of “am I not?”
Further, some say “ain’t” may have first come about as an attempt to contract “am I not” and later became used colloquially in lieu of “are not” and “has not.” Ain’t that something?
There is also a theory that “amn’t” made appearances as “an’t” in 18th century texts but, when pronounced by the British, sounded more like “ahnt” and later became “aren’t.”
I’ll bet there are readers who know the answer to this mystery and would be willing to share it with the rest of us.
Aren’t I just opening up a can of worms?
Word Nymph 
No, your sources are wrong! “Aren’t I” *is* correct, certainly for speech and most informal writing. Clearly, formal writing doesn’t count – but most things don’t count in formal writing anyway. If you grew up in the south of England, “aren’t I” is the default, if trying to be a little fresh (i.e. forward, presumptuous), and “am I not” is the emphatic form. Your mileage may vary, but applies if the person grew up there in the 1960s and ’70s.
“Logic would dictate …” – and that’s exactly what a lot of language analysis is fallen on. You and I know we don’t have to be brain surgeons to realise that logic goes only so far before it breaks down completely (e.g. Sapir-Worf Hypothesis a.k.a. B[w]anker’s Syndrome).
No can of worms here. I’m not flaming you – I don’t flame nice people.
In British English, at least around London, you had/have the oral compromise of “a’nt” (pronounced aiy-nt), which for most people needs to delving into if the contraction is from am or are.
I don’t know about Scottish speakers, but my Irish parents used “amn’t I” frequently. Another “English English” contraction to ponder: “mayn’t I?” as in “If I do all my homework, mayn’t I watch TV?”
I hadn’t thought of these two expressions in a long time: I shall start using them immediately. I may, mayn’t I?
Your surely may. I might as well. I like it.
“Mayn’t” is old-fashioned (really old fashioned even in England). My own experience was hearing it only in the north of England (Lancashire and the now-defunct Westmorland) and mostly from rural people when I was very, very young.