新世界ザルにおける「こころ」の理解 (特集 : 「心の科学」)

Abstract

In this article, we discuss the evolution of human intelligence from a standpoint of comparative cognitive science. The "social intelligence hypothesis", proposed by Byrne and Whiten (1988), suggests that the outstanding intelligence of humans has evolved as an adaptation to complex social environment with social tactics such replete as alliance, deception and cooperation. According to this hypothesis, the extraordinarily well-developed physical intelligence of humans that permits counting, mathematics, categorization, tool use, etc. has its roots in social intelligence. Here we investigated social intelligence of a New World monkey species, tufted capuchin monkeys (Cebus apella). This species is known to show excellent physical intelligence in tool use and as aides for quadriplegic people. Thus the "social intelligence hypothesis" predicts that they should also show good social intelligence. First, we examined whether they could spontaneously deceive other individuals. Pairs of capuchin monkeys, one dominant and the other subordinate, faced each other in a food-competition situation. Two out of four subordinate individuals started to "deceive" the dominant monkey by providing false information about the location of food or by simply withholding the information. However, the results were not clear enough to indicate whether the "deceptive" behavior was intentional. The next experiment investigated whether they could spontaneously cooperate with other individuals. Monkeys were first trained to perform the necessary sequence of actions to obtain rewards by themselves. Then, the monkeys were tested in pairs. We asked whether they could spontaneously divide the action sequence between the two in a cooperation task where they could never obtain rewards without the partner's help. All pairs spontaneously solved this task. This cooperative behavior was maintained even if only one of the pair was rewarded at a time, alternating across trials. We next investigated some fundamental abilities needed to understand others' intentions. In these experiments, we showed that capuchin monkeys could learn to recognize others' knowledge states, expect others' next action by observing body orientation, and recognize a social context in which a particular behavior of the experimenter predicted her next behavior. These results suggest that capuchin monkeys have at least several fundamental elements of social intelligence. The results of these studies are consistent with the "social intelligence hypothesis" ; capuchin monkeys have well-developed social intelligence as well as physical intelligence. Our results showed that at least one species of New World monkeys shares common fundamental social abilities with humans. This evidence suggests that the origin of human social abilities could date back to the common ancestor of humans and New World monkeys. In perspective for the future, we reason that intentionally performing deceptive or cooperative acts may require recognition of the other's disposition or personality, along with the social context. In order to obtain benefits from deceptive or cooperative behavior, the agents must take the other's behavioral patterns into account. Humans often infer others' personalities, characters or dispositions by observing their behaviors. This ability enables us to predict others' behavior accurately in novel situations. Thus, the evolution of this ability may have contributed to the development of intentional deception and altruistic cooperative behavior. Our preliminary data shows that a capuchin monkey changed his strategy according to the identity of the partner in a social learning situation. This might lead to a rudimentary form of elaborate social tactics such as "trust" or "distrust".