We study how the mechanisms of choice influence preferences when animals face more than 2 alternatives simultaneously. Choice mechanisms can be hierarchical (if alternatives are assigned to categories by their similarity and choice is between categories) or simultaneous (if options enter the choice process individually, each with its own value). The latter, although simpler, can lead to counterintuitive outcomes because expressed preference between options depends not only on the kinds of

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... s present but also on the number of exemplars within each kind, so that decision makers have a higher probability of picking an option of a given class when exemplars in this class are common. Higher preference for commoner options has indeed been shown in humans, and if present in animals, it would affect many choice domains, including prey and mate choice. We studied the problem using starlings making risk-sensitive choices. Subjects chose between a risky option and 1 (in binary choices) or 2 (in trinary choices) fixed options that were identifiable as distinct but were identical in reward rate and had no variance. Preference between the risky and each fixed option was unaltered between binary and trinary contexts, but subjects chose a higher proportion of the fixed kind when this was represented by 2 rather than 1 distinct food sources. This means subjects were objectively risk prone in binary and risk averse in trinary contexts. These results fit accounts based on learning principles, but contradict the expectations of functional models of choice, including risk-sensitivity theory.