Why be nice? Psychological constraints on the evolution of cooperation

Animals often aid others without gaining any immediate benefits. Although these acts seem to reduce the donor's fitness, they are only apparently altruistic. Donors typically help because they or their kin receive future benefits or avoid costly punishment. Reciprocal altruism--alternating the roles of donor and recipient--has been a well-studied form of cooperation among non-kin because of its intuitive appeal in explaining human cooperation. Despite immense theoretical interest, little empirical evidence substantiates the biological importance of reciprocal altruism in non-human animals. We propose that this is because psychological mechanisms constrain its application in cooperative contexts. In particular, we contend that cognitive limitations such as temporal discounting, numerical discrimination and memory make reciprocity difficult for animals.     

Mellow Monday and furious Friday: The approach-related link between anger and time representation

Time can be represented spatially in two prevalent metaphors; ego-moving has the self moving “forward” towards the future while time-moving has the future moving “forward” towards the self. Anger also is represented spatially by an approach-related motivation. Because time and anger share an approach-related spatial representation, we hypothesised a link between anger and the ego-moving time perspective. In Study 1, participants naturally adopting an ego-moving representation of time had higher trait anger than those adopting a time-moving representation. Study 2 showed that processing an angry event (vs. an emotionally neutral event) predicted more ego-moving spatial interpretations of time. In Study 3, a scheduling task priming ego-moving (vs. time-moving) time representation prompted higher state anger. Our results reveal a novel bi-directional link between the seemingly unrelated but similarly embodied abstract domains of anger and time.     

Grammatical pattern learning by human infants and cotton-top tamarin monkeys

There is a surprising degree of overlapping structure evident across the languages of the world. One factor leading to cross-linguistic similarities may be constraints on human learning abilities. Linguistic structures that are easier for infants to learn should predominate in human languages. If correct, then (a) human infants should more readily acquire structures that are consistent with the form of natural language, whereas (b) non-human primates' patterns of learning should be less tightly linked to the structure of human languages. Prior experiments have not directly compared laboratory-based learning of grammatical structures by human infants and non-human primates, especially under comparable testing conditions and with similar materials. Five experiments with 12-month-old human infants and adult cotton-top tamarin monkeys addressed these predictions, employing comparable methods (familiarization-discrimination) and materials. Infants rapidly acquired complex grammatical structures by using statistically predictive patterns, failing to learn structures that lacked such patterns. In contrast, the tamarins only exploited predictive patterns when learning relatively simple grammatical structures. Infant learning abilities may serve both to facilitate natural language acquisition and to impose constraints on the structure of human languages.     

Action comprehension in non-human primates: motor simulation or inferential reasoning?

Some argue that action comprehension is intimately connected with the observer's own motor capacities, whereas others argue that action comprehension depends on non-motor inferential mechanisms. We address this debate by reviewing comparative studies that license four conclusions: monkeys and apes extract the meaning of an action (i) by going beyond the surface properties of actions, attributing goals and intentions to the agent; (ii) by using environmental information to infer when actions are rational; (iii) by making predictions about an agent's goal, and the most probable action to obtain the goal given environmental constraints; (iv) in situations in which they are physiologically incapable of producing the actions. Motor theories are, thus, insufficient to account for primate action comprehension in the absence of inferential mechanisms.     

Rule learning by cotton-top tamarins

Previous work suggests that human infants are capable of rapidly generalizing patterns that have been characterized as abstract algebraic rules (Science 283 (1999) 77), a process that may play a pivotal role in language acquisition. Here we explore whether this capacity is uniquely human and evolved specifically for the computational problems associated with language, or whether this mechanism is shared with other species, and therefore evolved for problems other than language. We used the same materials and methods that were originally employed in tests of human infants to assess whether cotton-top tamarin monkeys can extract abstract algebraic rules. Specifically, we habituated subjects to sequences of consonant-vowel syllables that followed one of two patterns, AAB (e.g. wi wi di) or ABB (le we we). Following habituation, we presented subjects with two novel test items, one with the same pattern as that presented during habituation and one with a different pattern. Like human infants, tamarins were more likely to dishabituate to the test item with a different pattern. We conclude that the capacity to generalize rule-like patterns, at least at the level demonstrated, did not evolve specifically for language acquisition, though it remains possible that infants might use such rules during language acquisition.     

The relationship between problem solving and inhibitory control: cotton-top tamarin (Saguinus oedipus) performance on a reversed contingency task

To explore the relationship between problem solving and inhibitory control, the authors present 4 experiments on cotton-top tamarins (Saguinus oedipus) using a reverse-reward contingency task. In Experiment 1, 1 group of tamarins was given a choice between a small and a large quantity of food. Whichever quantity the tamarins reached for first, they received the alternative. The tamarins consistently picked the larger quantity, thereby receiving the smaller. A 2nd group of tamarins was given the same task, except that if they reached for the larger quantity of food, they received nothing. The tamarins continued to pick the larger quantity, even though this resulted in no food. In addition, most of the tamarins continued to pick the larger quantity even when the food payoff for choosing the smaller quantity was increased (Experiment 2) or when the visual salience of the food was reduced (Experiment 3). Experiment 4 was based on the finding that chimpanzees (Pan troglodytes) that have been trained on the concept of number can solve the reversed contingency task if the food is replaced by Arabic numerals. With the help of a color association, and a higher cost incurred by picking the color associated with 3 food items, the tamarins learned to pick the color associated with 1 food item. These results are compared with those obtained from studies of other primate species, highlighting the importance of comparative studies of problem solving that use comparable methods.     

Searching for food in the wild: a nonhuman rimate’s expectations about invisible displacement

Five experiments involving invisible displacements were run on a population of semi‐free‐ranging rhesus monkeys (Macaca mulatta). The goal of these experiments was to assess, without training, the kinds of expectations individuals spontaneously set up when an object has moved out of sight. The first experiment, modeled after studies of human infants and children, involved a table with one box on the top surface and a second box lined up below on the ground. An occluder was placed in front of the table, blocking the subject’s view. A piece of food was then dropped behind the occluder, above the top box. The presenter then removed the occluder, walked away, and allowed the subject to approach. Consistently, subjects searched in the incorrect bottom box. This error can be interpreted as a failure to understand solidity, containment, or some other factor. It can also be interpreted as an error guided by a gravity bias, i.e. an expectation that all falling objects fall straight down or to the lowest point. Experiments 2–5 tested these alternative hypotheses. Results show that rhesus monkeys do not have an inherent bottom box bias, are not avoiding the top box, and do recognize that in some contexts boxes can contain or hold food. Thus, for example, when the two boxes are placed on the ground, one in front of the other, and occluded, subjects search in the near box after a piece of food has been rolled behind the occluder (horizontal trajectory). This shows that rhesus can solve an invisible displacement problem that involves solid containers, where one container blocks travel to the other container. We conclude that the rhesus monkey’s error in Experiment 1 is guided by an expectation that all falling objects fall straight down or, at least, to the lowest point. This expectation represents a limitation of their knowledge of physical objects and events.     

The role of landmarks in cotton-top tamarin spatial foraging: evidence for geometric and non-geometric features

We report experiments on captive cotton-top tamarins (Saguinus oedipus) designed to explore two components of spatial foraging. First, do tamarins have the capacity to extract geometric information concerning the spatial relationship between a landmark and a piece of food located above or below it? Second, when tamarins use a landmark to find a target location, what non-geometric features of the landmark do they encode? To explore these problems, we created an artificial jungle environment and trained subjects to find food either above or below a target object (i.e., landmark). Once subjects successfully located the food, we transformed various features associated with the landmark, including its color, orientation, and shape; we also manipulated the landmark-food reward distance, the overall shape of the jungle, and the number and position of landmarks. Results showed that the tamarins' success in finding the food reward was not affected by landmark color, orientation, number, or overall shape of the jungle, suggesting that with respect to the particular test conditions, these features are not relevant to the representation of a landmark. Subjects also generalized to novel landmark-food distances, suggesting that they had integrated geometric (i.e., above/below) with non-geometric (i.e., color/shape) features. Performance was negatively affected by changes to the shape of the landmark, indicating that this feature is critical to the representation of a landmark. Accepted after revision: 7 August 2001 Electronic Publication