It turns out that Tit for Tat remains a very robust strategy, despite its apparent lack of sophistication, in prisoner-dilemma trials.
It's a very robust strategy because of its apparent lack of sophistication. By being easy to detect and predict, it causes its opponents to chose cooperation. Since you do better in PD if your opponent cooperates, a behavior that elicits cooperation is more effective than one that doesn't.
You might say It's effective at being affective.
Check it out here.
Brief summary: Dawkins demonstrates a classic "Prisoner Dilemma AI tournament". No big surprise to us today, but at the time the revelation that Tit for Tat is one of -- if not *the* -- most effective strateg(y|ies) was a surprising result. He goes on to demonstrate animals employing the Tit for Tat strategy. Assumptions of generosity, with vengefulness, appear to be strongly selected for.