How costs affect preferences: Experiments on state dependence, hedonic state and within-trial contrast in starlings
Aw JM., Vasconcelos M., Kacelnik A.
We examined how starlings, Sturnus vulgaris, adjust preferences to retrospective sunk costs in either time or work. Ideal decision-makers disregard sunk costs, but under some circumstances animals, like humans, prefer normally costlier rewards when they do not have to pay the costs. We argue that a possible explanation is state-dependent valuation learning (SDVL). The argument is that subjective value (hedonic state or fitness) is a decelerated function of energetic state, and energetic costs displace the subject's state towards conditions where the function is steeper and the subjective value of rewards is enhanced. We compared SDVL with within-trial contrast (WTC). WTC says that experiencing a food reward brings subjects to a baseline, constant, hedonic state, so that being in a more negative prereward mood causes a bigger positive displacement. SDVL focuses on energetic costs, while WTC focuses on any emotional dimension. We carried out three experiments that differed in the salience and dimension of the cost (time or work) and found overvaluation of costlier outcomes when the retrospective cost involved work (locomotory activity) but not when it involved only delays. When work was used as a cost, the effects of cost on preference showed strong stochastic transitivity. Our findings are consistent with studies in which energetic state was altered by deprivation rather than cost, and may help to explain why studies of WTC have had unreliable outcomes. © 2011 The Association for the Study of Animal Behaviour.