[waterhouse]

Yup. In real situations, one element is that sometimes things are misunderstood, and in particular sometimes collaborating is mistaken for defecting, so if all sides are playing pure tit-for-tat, then one can end up in a defect-defect loop forever (or, I guess, until the opposite misinterpretation occurs). Therefore, an element of forgiveness (some percent chance that you'll respond to a defect with a cooperate) can be helpful.