We seek an evolutionary explanation for why in some situations humans maintain either optimistic or pessimistic attitudes toward uncertainty and are ignorant to relevant aspects of their environment. Players in strategic games face Knightian uncertainty about opponents’ actions and maximize individually their Choquet expected utility with respect to neo-additive capacities allowing for both an optimistic or pessimistic attitude toward uncertainty as well as ignorance to strategic dependencies. An optimist overweighs good outcomes. A complete ignorant never reacts to opponents’ changes of (...) actions. We focus on sub- and supermodular aggregative games and provide monotone comparative statics w.r.t. optimism/pessimism. With qualifications, we show that in finite populations, optimistic complete ignorance is evolutionary stable and yields a strategic advantage in submodular games with aggregate externalities. Moreover, this evolutionary stable preference leads to Walrasian behavior in these classes of games. (shrink)