So what is really built into the person is a strategy: Play tit for tat. Do to others what they do unto you. Specifically, the tit-for-tat strategy is to be nice on the first round of interaction; but after that, do to your partner whatever your partner did to you on the previous round.10 Tit for tat takes us way beyond kin altruism. It opens the possibility of forming cooperative relationships with strangers.
