The optimal decision in the Prisoner's Dilemma is to defect, but in the iterated version, where multiple Dilemmas occur and people remember previous results, Tit-For-Tat is optimal. The real world is even less reminiscent of the Dilemma, so it's not at all clear that the Dilemma's conclusion applies.

(Tit-For-Tat: Prefer cooperating, but if the other person defected on the previous turn, defect on the current turn.)