The evolutionary stability of optimism, pessimism, and complete ignorance

We provide an evolutionary foundation to evidence that in some situations humans maintain either optimistic or pessimistic attitudes towards uncertainty and are ignorant to relevant aspects of the environment. Players in strategic games face Knightian uncertainty about opponents' actions and maximize individually their Choquet expected utility with respect to neo-additive capacities (Chateauneuf, Eichberger, and Grant, 2007) allowing for both an optimistic or pessimistic attitude towards uncertainty as well as ignorance to strategic dependencies. An optimist (resp. pessimist) overweights good (resp. bad) outcomes. A complete ignorant never reacts to opponents' changes of actions. With qualifications we show that in finite populations optimistic (resp. pessimistic) complete ignorance is evolutionary stable and yields a strategic advantage in submodular (resp. supermodular) games with aggregate externalities. Moreover, this evolutionary stable preference leads to Walrasian behavior in these classes of games.