You’ve just finished cooking dinner when your roommate enters the kitchen. Suspecting that she doesn’t know the stove is still hot, you warn her before she gets too close.
We can often recognize the differences between what we know and what others know, allowing us to predict what others might think and do. Most humans demonstrate this ability by the age of four. But is this ability unique to humans, or could other animals also predict others’ beliefs?
In a recent study led by Isao Hasegawa, a physiologist at Niigata University School of Medicine in Japan, researchers tested whether a monkey could guess what a human believes. They tracked the monkey’s eyes to infer its expectations. When monkeys (and people) expect someone to take a particular action, they tend to look toward relevant objects or locations in the environment. Where a monkey looks can reveal what it expects someone else to do.
The researchers took advantage of this behavior while a monkey watched videos involving unexpected actions. In the videos, a human actor (the “opponent”) hides a toy under an overturned box in the presence of another actor (the “searcher”). The searcher then leaves the room. While the searcher is absent, the opponent moves the toy to a new hiding place under a second overturned box, and then removes the toy entirely while running away. Finally, the searcher returns to the room to retrieve the hidden toy. The searcher doesn’t know that the opponent has removed the toy. But the monkey, having seen the entire video, knows that the toy is gone.
The researchers wanted to know if the monkey would understand that the searcher’s beliefs differ from reality. They tracked the monkey’s eyes to determine where the monkey believed the searcher would look for the toy. If the monkey tended to look at the original hiding place, this would suggest that it understood the searcher’s false beliefs about the toy’s location. However, if the monkey tended to look at the second box or elsewhere in the scene, this would suggest that the monkey did not fully grasp that the searcher’s beliefs are different from its own.
They found that monkeys were likely to look at the first box, where the searcher believed the toy was still hidden, indicating that monkeys expected the searcher to look for the toy under that box. These results suggest that monkeys can understand others’ false beliefs and predict their actions accordingly, just like humans. Perhaps humans have more in common with other animals than we think.