[Tversky and Fox] "Decisions under uncertainty depend not only on the degree of uncertainty but also on its source, as illustrated by Ellsberg's observation of ambiguity aversion. Ambiguity aversion is produced by a comparison with less ambiguous events or with more knowledgeable individuals. This hypothesis is supported in a series of studies showing that ambiguity aversion, present in a comparative context in which a person evaluates both clear and vague prospects, seems to disappear in a noncomparative context in which a person evaluates only one of these prospects in isolation".

Ellsberg paradox, Wikipedia
The Ellsberg paradox is a paradox in decision theory and experimental economics in which people's choices violate the expected utility hypothesis. It is generally taken to be evidence for ambiguity aversion. The paradox was first discovered by Daniel Ellsberg.

Ambiguity Aversion, Comparative Ignorance, and Decision Context, Fox and Weber
"Most choices in life have uncertain consequences. Decisions to undergo surgery, invest in a mutual fund, or purchase insurance must be made without knowing in advance whether the operation will be successful, the fund will outperform the market, or there will be a need to make a claim on the policy. Decision makers are required to judge the likelihood of relevant events for themselves based on computation, intuition, and/or hearsay, with some degree of imprecision or vagueness. The contrast between clear and vague probabilities was first discussed by Knight (1921), who distinguished between risk, which can be represented by precise probabilities, and (unmeasurable) uncertainty, which cannot. Interest in the problem of vague probabilities was revived by Ellsberg (1961), who argued that people generally prefer to bet on known rather than unknown probabilities. Ellsberg's simplest demonstration involves two urns, each containing 100 balls. The first urn contains 50 red balls and 50 black balls, whereas the second contains red and black balls in an unknown proportion. When asked to bet on a blind draw from an urn, most people express no particular color preference, but they would rather bet on the clear (50- 50) urn than on the vague urn. This pattern of preferences violates expected utility theory because the subjective probabilities of red and black, derived from preferences, cannot sum to one for both urns. Ellsberg's own interpretation was that in addition to the utility of outcomes and the probability of events determining them, decision makers consider a "third dimension", which he called ambiguity. A more recent stream of research emphasizes instead the decision maker's confidence in his or her knowledge or information. Contrary to the ambiguity aversion hypothesis, Heath and Tversky (1991) found that people prefer to bet on their vague beliefs in situations where they feel especially competent or knowledgeable, though they prefer to bet on chance when they do not. The competence hypothesis suggests that ambiguity aversion is governed by the decision maker's appraisal of his or her knowledge of relevant events rather than some second-order measure of probability vagueness. Furthermore, Fox and Tversky (1995) argued that perceived competence only affects decisions to the extent that this dimension is brought to mind. According to their Comparative Ignorance hypothesis, ambiguity aversion is driven by the comparison with more familiar events or more knowledgeable individuals, and diminishes or disappears in the absence of such a comparison. This represents a sharp break from previous accounts of decision under uncertainty because it asserts that decisions are influenced by the cognitive context in which the decision maker finds him or herself so that a particular uncertain prospect may be more or less attractive depending on whether or not a contrasting state of knowledge is salient. We extend the comparative ignorance hypothesis by documenting four new ways in which decision context can affect willingness to act under uncertainty. First, people find uncertain bets more attractive when preceded by questions about less familiar items than when preceded by questions about more familiar items. Second, the preference to bet on more familiar domains is less pronounced for the first domain evaluated on a survey than for later domains. Third, people find bets less attractive when they are provided with diagnostic information that they do not know how to use, compared to when they are provided with no such information. Finally, people are sensitive to the relative competence of their counterpart when playing a simple competitive ("matching pennies") game, but not when playing a noncompetitive (coordination) game that has the same mixed strategy Nash equilibrium".