I like to point out that tit-for-tat is not the fittest strategy under all circumstances, but only in tournaments where strategy can't be tailored to asymmetrical information (called communication in the real world).

The dilemma is that the line between cooperate/shirk is not that clear and aren't signaled during the game alone.

Make sure your competitor knows you are cooperating, and that you will be irrationally glad to punish shirking, and are commited to keep on punishing with brutal intensity.

Thanks for the post.