Capuchin monkeys (Sapajus [Cebus] apella) are more susceptible to contrast than to decoy and social context effects

Theories of optimal decision-making typically assume that animals have consistent preferences among options. In reality, economic behaviour in humans and foraging behaviour in some animals is often susceptible to choice-irrelevant factors such as inferior options or conspecifics’ outcomes, but the evidence for primate decision-making is mixed. Unlike previous experiments, we assessed the relative magnitude of three context effects. Using a food preference paradigm, we varied the number of cereal pieces to determine how much a piece of food A was “worth” (50% choice) to each of 13 capuchins. We predicted that monkeys would devalue A in the contrast condition (when a higher-quality but unattainable food was present) and overvalue it in the decoy condition (when a smaller version of A was a third option) and social condition (when A, if unchosen, was given to a partner). Capuchins were 4 times less likely to choose A in the contrast condition, but 2 to 3 times more likely to choose it in the decoy and social conditions. When carefully accounting for initial preferences, we found that these primates, like humans, are sensitive to context effects. This suggests that these biases are evolved and impacts how we think about them in humans.