It seems to me that you're actually asking a question, either way (Are you not smart?") and that question implies that there is a question about the person's intelligence. If it's a sincere comment, you would simply say "You are smart" or "You are very smart" or some sort of affirmative comment as opposed to a questioning phrase. I know we can get into all manner of discussion about the phrase "aren't you?" but in this context, it sounds sarcastic to my ears.Is saying "Aren't you smart?" really sarcastic? One book I'm reading says it is, and should be "You are smart, aren't you."