I'm not sure about "am I not", but as far as I remember, "amn't I" is mostly used in Scotland and Northern Ireland, while "aren't I" is usual in England, America, and Australia.
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CalifJim...both are correct and that most grammarians will insist that "Jessica is prettier than I" is better, even though many important authors have used "... than me", which is also what people very often say in everyday conversations, and which is contrary to what the grammarians recommend.See also and
New2grammarShouldn't it be "I'm taller than him"?I'm taller than he is.