Comparative statics of altruism and spite

► Altruism modifies playersʼ payoffs in a game by mixing them with some social payoff. ► This modification may paradoxically result in a lower equilibrium social payoff. ► However, this can only occur if the equilibrium strategies involved are unstable. ► 'Stability' in this result refers to a novel, general notion of static stability. ► It generalizes a number of other, familiar notions of static stability such as ESS.