If you raise 15% of hands from MP, and a person threebets you 20% of the time you raise from MP, is he exploiting you if you play with 4% of those hands (folding, what, 70% of the time??) No.

I think people in this thread are somewhat lost. Threebetting a large frequency isn't an inherently good move and versus a lot of players isn't a good thing to do.

Jyms, just don't play anything but AK/JJ to threebets from UTG, and call with something like AJ+, KQ, 88 from the CO and BU (versus lighter threebettors) and you'll be golden. O

Obviously, the real rule is you adjust your calling/fourbet range based on:
1. With what frequency and hands opp is threebetting.
2. How often your opening from that position preflop

If someone played 1/1, aka only opened AA/KK preflop and folded the rest, would threebetting this person with anything but AA ever be correct? No. Thats a microcosmic point representing the real macrocosmic concept.