"Game theory explained" with too many assumptions. Such as that punishment is so costly that it creates another problem of the commons, as the punishers (who are a subset of the cooperators) get less our of it than the non-punishing cooperators. I'd say this is wrong: in repeated games punishment is not a cost but an investment. A threat must be carried out in order to be credible in the long run, and thus a cooperator who punishes a defector (or a 'joker' for that matter) is better positioned in subsequent games to be treated fairly.