There is great interest in the evolution of economic behavior. In typical studies, species are asked to play one of a series of economic games, derived from game theory, and their responses are compared. The advantage of this approach is the relative level of consistency and control that emerges from the games themselves; however, in the typical experiment, procedures and conditions differ widely, particularly between humans and other species. Thus, in the current study, we investigated how three primate species, capuchin monkeys, chimpanzees, and humans, played the Assurance (or Stag Hunt) game using procedures that were, to the best of our ability, the same across species, particularly with respect to training and pretesting. Our goal was to determine what, if any, differences existed in the ways in which these species made decisions in this game. We hypothesized differences along phylogenetic lines, which we found. However, the species were more similar than might be expected. In particular, humans who played using "nonhuman primate-friendly" rules did not behave as is typical. Thus, we find evidence for similarity in decision-making processes across the order Primates. These results indicate that such comparative studies are possible and, moreover, that in any comparison rating species' relative abilities, extreme care must be taken in ensuring that one species does not have an advantage over the others due to methodological procedures.cooperation | coordination | comparative behavior | evolution of behavior R ecent advances in the study of economic decision making have fundamentally altered how we view the science of economics. Beginning with experimental economics (1) and continuing in more recently emerging fields such as neuroeconomics (2, 3), there has been a much more scientific approach to understanding how humans make decisions in economic contexts. Most recently, there has emerged an interest in understanding the evolution of human decision making, primarily as studied using a comparative approach. Although studies of rats and pigeons emerged many decades ago, a recent surge with additional species has provided even more data relevant to social scientists interested in decision making.Although game theory has been used independently in behavioral ecology for decades (4), it is only recently that human economic games have been used extensively to address decisionmaking behavior (5, 6) and underlying neural activity (7). There are certainly reasons to think that humans and other primates might be similar in their decision-making abilities. Other primates are our closest living relatives-we share a common ancestor with chimpanzees within approximately the last 6 million y (8)-so there is a high likelihood of homology. Even though this does not mean identical decision making, it implies similarity in the underlying structures. Nonhuman primates also show many of the same cognitive skills, and even biases (9, 10), as humans. Alternately, though, humans are distinct from other primates, and even a few mil...

The authors investigated the role that entropy measures, discriminative cues, and symbolic knowledge play for rhesus monkeys (Macaca mulatta) in the acquisition of the concepts of same and different for use in a computerized relational matching-to-sample task. After repeatedly failing to perceive relations between pairs of stimuli in a 2-choice discrimination paradigm, monkeys rapidly learned to discriminate between 8-element arrays. Subsequent tests with smaller arrays, however, suggested that, although important for the initial acquisition of the concept, entropy is not a variable on which monkeys are dependent. Not only do monkeys choose a corresponding relational pair in the presence of a cue, but they also choose the cue itself in the presence of the relational pair--in essence, labeling those relations. Subsequent failure in the judgment of relations-between-relations, however, suggests that perhaps a qualitatively different cognitive component exists that prevents monkeys from behaving analogically.

Recent evidence indicates that oxytocin plays an important role in promoting prosocial behaviour amongst humans and other species. We tested whether oxytocin affected cooperation and foodsharing in capuchin monkeys, a highly cooperative New World primate. Subjects received either 2IU oxytocin or an inert adjuvent intranasally prior to each session. Oxytocin influenced food sharing in capuchins in ways we did not anticipate. Recipients were less likely to passively acquire food from possessors when either individual had received OT than in the control, and also spent less time in proximity to their partner. Passive food sharing requires proximity, and oxytocin decreased the capuchins' typical congregating behaviour, apparently resulting in decreased sharing. We propose that the likely mechanism for increased social distance is the known anxiolytic effect of oxytocin. Our results indicate a need to consider how oxytocin affects the context of interactions and interacts with modes of sociality unique to each species.

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Thus far, language- and token-trained apes (e.g., D. Premack, 1976; R. K. R. Thompson, D. L. Oden, & S. T. Boysen, 1997) have provided the best evidence that nonhuman animals can solve, complete, and construct analogies, thus implicating symbolic representation as the mechanism enabling the phenomenon. In this study, the authors examined the role of stimulus meaning in the analogical reasoning abilities of three different primate species. Humans (Homo sapiens), chimpanzees (Pan troglodytes), and rhesus monkeys (Macaca mulatta) completed the same relational matching-to-sample (RMTS) tasks with both meaningful and nonmeaningful stimuli. This discrimination of relations-between-relations serves as the basis for analogical reasoning. Meaningfulness facilitated the acquisition of analogical matching for human participants, whereas individual differences among the chimpanzees suggest that meaning can either enable or hinder their ability to complete analogies. Rhesus monkeys did not succeed in the RMTS task regardless of stimulus meaning, suggesting that their ability to reason analogically, if present at all, may be dependent on a dimension other than the representational value of stimuli.