"When players were confronted with a free-rider, almost everyone put themselves at the upper end of the anger scale. Fehr and Gachter describe these emotions as a “proximate mechanism”. This means that evolution has built into the human mind a hatred of free-riders and cheaters, which activates anger when we confront people acting like this – and it is this anger which prompts altruistic punishment. In this way, the emotion is evolution’s way of getting us to overcome our short-term self-interest and encourage collective social life"