After screening for attentiveness and comprehension, we present subjects with Ellsberg's (1961) two-urn problem using essentially equivalent but representationally complex matrices. Highcomprehension subjects exhibit rates of ambiguity aversion typical of the standard two-urn problem, while low-comprehension subjects appear to randomize. In screening, we classify subjects as “probability-minded” or “ambiguity-minded”, depending on whether they assign probabilities to draws from a card deck of unknown composition. Among high-comprehension subjects, “mindedness” explains twenty times more variation in ambiguity attitudes than all other demographic characteristics combined. Compared with their “probability-minded” counterparts, “ambiguity-minded” subjects are younger and more educated, analytic, and reflective about their choices.
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