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I, too, tended to come out with about 30 points no matter what I did, although I got more when I alternated cooperation with occasional cheating. I always ended up with almost exactly the same number of coins as Serendip, though, since each time I cooperated after having cheated, Serendip would take advantage of me. Even if it did help to be tricky at times, the best strategy I found also increased my partner's gains.

My relationship with my roommate is a non-zero sum game as well-- it is in both of our best interest to cooperate, even when that requires that we make compromises in our living space. If one of us enforced our will on every situation, she'd get what she wanted, but the other roommate would be miserable; if we decided to fight over everything, neither of us would win.