Monkeys and Beliefs

Rhesus macaques from the Cayo Santiago population. Photo courtesy of Laurie Santos.

What makes us human? Scientists have added an empirical twist to this age-old philosophical inquiry by rephrasing the question and asking: how are humans different from other species? Of course, the answers that we find most fascinating are not about the body – they’re about the mind.

Yale Professor of Psychology Laurie Santos tackles the question of what makes us human by studying the minds of our closest relatives: primates. Since an extraordinary part of human mental life is our ability to reason about the minds of others, Santos has long been interested in exploring what primates know about the minds of others. Studies conducted in recent years suggest that monkeys are indeed able to understand what others see and what others know. For instance, a subordinate chimpanzee will only take a piece of food that a dominant monkey cannot see or does not know about.

But, there is more to understanding the minds of others than simply determining whether or not someone has access to a certain piece of information about the world. In a paper recently published in Developmental Science, Santos and colleagues in the Yale Department of Psychology explain that “in order to represent mental states, one must first recognize that mental states are psychological in nature. Unlike real states of the world, mental states exist inside an individual’s mind and, thus, may be incongruent with the genuine state of the world.” Researchers have therefore become very interested in how monkeys behave when facing someone whose mental state is different from the actual state of the world – that is, when someone has a false belief.

Santos and colleagues studied the Rhesus macaques (Macaca mulatta) in the Cayo Santiago population using a looking-time measure based on the fact that monkeys (like human infants) look longer at unexpected events as compared to expected events. First, an experimenter carried a simple testing apparatus: a foam core stage with a box on each side (“Box A” and “Box B”) and a plastic lemon able to roll between the boxes (on a track controlled by the experimenter). In the first task, the experimenter watched as the lemon rolled into Box A, and then she reached to search for the lemon. Monkeys looked longer when the experimenter failed to search Box A, indicating that the monkeys understood that the experimenter had knowledge of the lemon’s location.

In the second task, the experimenter watched as the lemon rolled into Box A. Then a hinged screen rose up to completely cover the face of the experimenter – clearly obstructing the experimenter’s view of the stage – and the lemon rolled across the stage into Box B. Monkeys apparently understood that the experimenter lacked knowledge of the lemon’s actual location, and did not expect the experimenter to search Box B. However, the monkeys were unable to recognize the experimenter’s false belief – rather than expecting the experimenter to search Box A, where the experimenter last saw the lemon, the monkeys did not demonstrate any expectation about where the experimenter would search. They looked equally long in both cases.

These studies support the idea that monkeys can represent the knowledge and ignorance of others, but not their beliefs. As Santos explained, “belief representations are more complex because you need to simulate the content of another individual’s thoughts.” Using the same methods employed in Santos’ experiment, it has been found that 15-month-old human infants are able to represent beliefs in exactly this complex way. This appreciation of the subjective nature of psychological experience is an important part of what makes us human.

Professor Santos with a rhesus macaque in Cayo Satiago. Photo courtesy of Laurie Santos.