The conventional wisdom is that, in a cash game (as opposed to a tournament), KK should practically never be folded before the flop. In Harrington on Holdem Volume I, Dan Harrington describes a time that he almost folded them, but then decided to call anyway. He claims to have never folded KK before the flop.
I've folded KK before the flop once in my life, back in college against my old roommate Ben. I now think it was a bad decision, although Ben insists that he did, indeed, have AA (I'm not entirely sure he's telling me the truth, though). Anyway, I almost folded KK again yesterday at the Rio, and after analyzing the situation more closely, I'm still not sure what I should have done.
Here's the situation. Playing 2-5 NL, I was in the small blind with about $800. The under-the-gun player, who had exactly $332 left, raised to $25. He was tight and rather passive, an I hadn't seen him do anything out of line in the couple of hours we had been there. So already I knew he had to have a premium hand.
The next player had over $1000 behind. He was loose and aggressive, and raised to $100. I figured him for AQ or better, TT or better. Everyone folded to me, and I looked down at KK. I decided that I very likely had the best hand, but that the UTG player also had to have an excellent hand, and was likely to put in the rest of his money if I just called. This was my best chance of getting the loose-aggressive player's money into the pot. So, instead of reraising, which is the usual play, I just cold-called the $100. The big blind folded, and, as expected, the UTG player went all-in for his last $232. He did this so confidently and uncaringly that I felt he very likely had AA.
The loose aggressive player spent a minute or so considering his play, and made the comment "I'm either going all-in or folding," essentially eliminating the chance he had AA... so at this point I was really hoping he pushes all-in. Alas, he folded, accidentally flashing two face cards that I thought were JJ (he confirmed this after the hand). In any case, he certainly was not holding an ace.
Now the action was back to me. I am reasonably confident the UTG player has AA, but can I really lay down KK? I was being offered 232 to 533 odds. This means I need to think I have about a 30% chance to win the hand to make calling correct. If he has AA I am only about 18 or 19% to win the hand. In my mind at the time, I was thinking that if he didn't have AA, I was about 80% to win. After further analysis, I think that if he didn't have AA, I was really only about 75% to win, but I was pretty close. (The 75% statistics was calculated with pokerstove, assuming the other possible hands he could have were 88-KK, ATs+, KQs, or AKo. Adding in a few hands like 78s doesn't change things much.) I decided to call, but expected to see him turn over AA... which is exactly what happened.
Anyway, doing the math shows that I needed to be 79% confident that my opponent was holding AA to make folding correct. Was I that confident? Uh... I dunno, I've always found it hard to put numbers on these things. I actually think I was about 80% sure he had AA. So, maybe folding would have been correct after all.