Right - if I recall, the story is that there were computer-simulated tournaments in which different strategies for playing the iterated prisoner's dilemma were

Right - if I recall, the story is that there were computer-simulated tournaments in which different strategies for playing the iterated prisoner's dilemma were pitted against each other, and tit-for-tat is the strategy that emerged to be the winner. So it's an empirical result showing that it is generally the most successful strategy, not mathematically shown to be optimal against every strategy.