In Experiment 1, rats choosing in an E maze preferred to release a rat standing in a pool of water to dry ground over a rat already standing on dry ground. Five additional experiments showed that the choosing rat’s preference for releasing the wet rat was maintained by two separable outcomes: (a) the social contact offered by the released rat, and (b) the reinforcing value of proximity to a pool of water. These results call into question Sato et al.’s (2015) claim to have demonstrated that a rat’s releasing of a wet rat to dry ground is empathically motivated.

In Experiment 1, six capuchins lifted a weight during a 10-minute session to receive a food piece. Across conditions, the weight was increased across six different amounts for three different food types. The number of food pieces obtained as a function of the weight lifted was fitted by a demand equation that is hypothesized to quantify food value. For most subjects, this analysis showed that the three food types differed little in value. In Experiment 2, these monkeys were given pairwise choices among these food types. In 13 of 18 comparisons, preferences at least equaled a 3-to-1 ratio; in seven comparisons, preference was absolute. There was no relation between values based on degree of preference versus values based on the demand equation. When choices in the present report were compared to similar data with these subjects from another study, between-study lability in preference emerged. This outcome contrasts with the finding in demand analysis that test-retest reliability is high. We attribute the unreliability and extreme assignment of value based on preference tests to high substitutability between foods. We suggest use of demand analysis instead of preference tests for studies that compare the values of different foods. A better strategy might be to avoid manipulating value by using different foods. Where possible, value should be manipulated by varying amounts of a single food type because, over an appropriate range, more food is consistently more valuable than less. Such an approach would be immune to problems in between-food substitutability.

Researchers in comparative psychology often use different food rewards in their studies, with food values defined by a pre-experimental preference test. While this technique rank orders food values, it provides limited information about value differences because preferences may reflect not only value differences, but also the degree to which one good may “substitute” for another (e.g., one food may substitute well for another food, but neither substitutes well for water). We propose scaling the value of food pairs by a third food that is less substitutable for either food offered in preference tests (cross-modal scaling). Here, Cebus monkeys chose between four pairwise alternatives: fruits A vs. B; cereal amount X vs. fruit A and cereal amount Y vs. fruit B where X and Y were adjusted to produce indifference between each cereal amount and each fruit; and cereal amounts X vs. Y. When choice was between perfect substitutes (different cereal amounts), preferences were nearly absolute; so too when choice was between close substitutes (fruits); however, when choice was between fruits and cereal amounts, preferences were more modest and less likely due to substitutability. These results suggest that scaling between-good value differences in terms of a third, less-substitutable good may be better than simple preference tests in defining between-good value differences.

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