Animals' Choices Over Uncertain Outcomes: Further Experimental Results

An experiment with rats was set up to test for fanning out and linearity of indifference curves in the unit probability triangle. After adapting standard Allais-type common ratio manipulations to rats, the results show clear and systematic Allais-type violations of the independence axiom of expected utility theory. While these manipulations produce fanning out in the southeast corner of the unit probability triangle, choices over prospects in the northwest portion of the triangle display fanning in. Further, fanning in is most pronounced when at least one of the prospects lies inside the triangle. Although these results violate the fanning out hypothesis, they do not disconfirm generalized expected utility theory. Tests of the betweenness axiom show systematic violations in favor of folded orderings, implying that indifference curves are convex rather than linear in the probabilities. These violations of the betweenness axiom were related to the failures of fanning out.