| Abstract
| - A decision-maker is said to have an ambiguous belief if it is not precise enough to be represented by a single probability distribution. The pervasive assumption in game theoretic models in economics is that players' beliefs are unambiguous. This paper argues, drawing on examples from economics and politics, that it may be illuminating, in instances, to model players as having ambiguous beliefs. Optimistic and pessimistic responses to ambiguity are formally modelled. We show that pessimism has the effect of increasing (decreasing) equilibrium prices under Cournot (Bertrand) competition. In addition the effects of ambiguity on peace-making are examined. It is shown that ambiguity may select equilibria in coordination games with multiple equilibria. Some comparative statics results are derived for the impact of ambiguity in games with strategic complements.
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