Apes prove it: You don’t have to be human to understand what someone else is thinking

With the help of an actor in a King Kong suit and infrared eye-trackers, researchers made a strong case that nonhuman primates can figure out what others are thinking. Previously, that ability was thought to be unique to humans. (Christopher Krupeny


Leave it to a guy in a King Kong suit to demonstrate that humans’ thinking abilities aren’t quite as special as we’d like to think.

Of all the creatures in the animal kingdom, only humans were given credit for being able to ascertain the unstated thoughts, beliefs and desires of others. (Of course, said credit was doled out by humans.) This is a skill of great evolutionary importance — in complex societies, it’s easier to get ahead if you can correctly anticipate the behavior of others.

A notable version of this skill is the ability to recognize when someone else believes something that’s false. If you can do this, it shows you actually understand what’s in someone else’s head, not merely how someone ought to react in a certain situation.


Scientists have tested this in children by having them watch a doll named Sally put a block in a box. After Sally leaves, another doll named Anne comes in and moves the block. Then the children are asked where Sally will look for it. Around the age of 4, kids figure out that Sally will check the box where she put the block, even though they know it’s the wrong place to look.

About a decade ago, researchers figured out a way to give this test to younger kids. They showed them the same saga of Sally and her hidden object, then used eye-tracking technology to see where they expected Sally to go. Lo and behold, toddlers who had just turned 2 were able to see that Sally would act on false beliefs.

These experiments gave scientists a great idea: If eye-tracking technology works with nonverbal humans, why not try it on our fellow great apes?

A team led by evolutionary anthropologist Christopher Krupenye at Duke University and comparative psychologist Fumihiro Kano of Kyoto University enlisted 41 chimpanzees, bonobos and orangutans. The researchers showed the apes a series of videos starring a regular person and a man dressed up as King Kong. In a variety of scenarios, King Kong would try to hide a rock-like object from the person. If the person saw what was going on, he’d find the rock in the expected place. If not, he didn’t.

To test whether the apes understood what was going on, the scientists showed them additional videos in which King Kong tried to hide in one of two haystacks. Sometimes the person saw where King Kong went, and sometimes he didn’t. Using an infrared eye-tracker, the researchers could see where the apes were looking — and thus, where they expected the human to go.

In a second experiment, the apes watched more videos of King Kong trying to hide a rock from the person. Again, the researchers used eye-trackers to see if the apes could anticipate what the person would do.

A man dressed in a King Kong suit tries to hide a rock-like object from a human, as an ape looks on. Eye-trackers allowed scientists to see whether the ape could figure out what the human was thinking. (Christopher Krupenye, Fumihiro Kano, MPI-EVA,


In both cases, the chimpanzees, bonobos and orangutans correctly anticipated the person’s actions — even when the person looked for King Kong or the rock in the wrong place. Some of the apes made mistakes, but overall they were significantly more likely to guess right than guess wrong, the researchers wrote in Friday’s edition of the journal Science.

Thanks to these results, the claim that only humans can ascertain the mental states of others “is starting to wobble,” primatologist Frans de Waal of the Yerkes National Primate Research Center at Emory University wrote in a commentary that accompanies the study.

This not-quite-uniquely-human ability probably arose in a hominid ancestor we share with our closest primate cousins, Krupenye and his colleagues wrote.

De Waal seconded that notion, saying the results highlight “the mental continuity between great apes and humans.”

It’s a useful reminder that humans shouldn’t be so quick to put themselves on a pedestal, he added.

“Reading others’ minds is beyond anybody’s capacity,” he wrote. “All we can do — and what apes apparently do in similar ways — is read bodies.”

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