Choice tests are commonly used to measure animals’ preferences, and the results of such tests are used to make recommendations regarding animal husbandry. An implicit assumption underlying the majority of choice tests is that the preferences obtained are independent of the set of options available in the test. This follows from two assumptions about the mechanisms of choice: first, that animals use absolute evaluation mechanisms to assign value to options, and second, that the probability of choosing an option is proportional to the ratio between the value of that option and the sum of the values of the other options available. However, if either of these assumptions is incorrect then preferences can differ depending on the composition of the choice set. In support of this concern, evidence from foraging animals shows that preferences can change when a third, less preferred option is added to a binary choice. These findings have implications for the design and interpretation of choice tests.
