If everyone behaved logically then the only sensible choice in a prisoners’ dilemma is to not co-operate with the other player as you are always better off by doing this. When this game is tested in experiments or seen in real-life, this isn’t what happens. Some people co-operate.
One explanation for this is that they haven’t understood the game properly but the level of co-operation is to high to be explained by this alone. In reality some people value co-operation more highly and so, for them, there are other factors than just the simple payoff of their decision.
In the case of a player in the standard prisoners’ dilemma, it may be that although the sentence they would receive is longer they would rather have that than be seen to be ‘grassing’ on the other person.
This doesn’t mean that the theory is a waste of time, just that people’s decisions in reality are not based on a single issue, there is always a balance of factors to be taken into account.
Part four of this post looks at what happens when the game is played lots of times rather than just as a one-off.