I gotta say the more I think about it, the more I agree with JS. My initial reaction to tit-for-tat made me overlook that the optimal strategy in IPD is to "forgive" (cooperate) even if he has defected any times in the past. Also the concept of treating others as I would want to be treated. If I was engaging in behavior that was causing other(s) harm, I would (in the long run) want to be corrected for it.