Monkeys represent others' knowledge but not their beliefs

Abstract

The capacity to reason about the false beliefs of others is classically considered the benchmark for a fully fledged understanding of the mental lives of others. Although much is known about the developmental origins of our understanding of others' beliefs, we still know much less about the evolutionary origins of this capacity. Here, we examine whether non-human primates - specifically, rhesus macaques (Macaca mulatta) - share this developmental achievement. We presented macaques with a looking-time measure of false belief understanding, one that had recently been developed for use with 15-month-old human infants. Like human infants, monkeys look longer when a human experimenter fails to search in the correct location when she has accurate knowledge. In contrast to infants, however, monkeys appear to make no prediction about how a human experimenter will act when she has a false belief. Across three studies, macaques' pattern of results is consistent with the view that monkeys can represent the knowledge and ignorance of others, but not their beliefs. The capacity to represent beliefs may therefore be a unique hallmark of human cognition.

Figure 1

The belief induction and test events used in Experiment 1. Across two conditions (different side and same side), the experimenter had a true belief about the lemon’s location and should be expected to reach to the box that actually contains the lemon.

Figure 2

Mean looking time Expected and Unexpected conditions for all experiments. Brackets show the standard error of the mean. Monkeys looked significantly longer in the Unexpected condition than the Expected condition in Experiments 1 and 3 (*p < .05, ** p < .01). There was no significant difference in looking time between conditions in Experiment 2.

Figure 3

The belief induction and test events used in Experiment 2. Across two conditions (different side and same side), the experimenter had a false belief about the lemon’s location and should be expected to reach to the incorrect box.

Figure 4

The belief induction and test events used in Experiment 3. Across two conditions (different side and same side), the experimenter knew where the lemon was hidden, but there was a short delay in which the experimenter’s view of the stage was occluded before her search action occurred. If the demand of the delay is inconsequential, monkeys should expect the experimenter to reach to the correct box.