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But “there’s a lot to unpack about what this task is really telling us,” she adds. One hallmark of our theory of mind is our ability to hold and compare two competing views of the world: a false belief, and the reality. There’s still no evidence that apes can do this. Sure, they might infer that the actor thinks the stone is in the wrong place, but can they simultaneously remember where the stone actually is? Are they even thinking about the stone at all? “There’s an inferential leap to this being similar to humans,” she says.
Kano anticipates other critiques. Skeptics could still argue that the apes are using a more complicated rule: Look where the actor last saw the object with a clear line of sight. The type of tasks he used—anticipatory-looking exercises—are also controversial. They’ve been used to demonstrate theory of mind in human infants, but several researchers recently tried to replicate those studies with mixed results. “We did replicate it with great apes though, which is quite strange,” says Kano.
The problem, he thinks, is that many researchers use videos that are incredibly boring, where actors impassively and slowly move objects between bowls. When he initially showed such videos to his apes, they just didn’t care. That’s why he brought out the Kong suit, and added an element of theft. “The apes want to see more action, and maybe even violence,” he says. “And they’re slightly irritated by this weird ape-looking guy.”
Still, even Kano isn’t entirely convinced by his own studies, and admits there’s plenty for room for doubt. On a scale of one to 10, he rates his confidence that apes have a full theory of mind at a six. “We started these series of experiments believing that apes have theory of mind,” he says, “but there are so many alternatives that we need to test.”
Read: The surprising complexity of animal memory
For example, others have argued that tracking beliefs about an object’s location is a relatively simple feat that falls short of the full-fledged theory of mind that we possess. It would be more impressive if the apes could understand someone’s beliefs about an object’s identity: What happens, for example, if the pilfered stone is swapped for something else? That’s Kano’s next project. “Given the remarkable ingenuity and competence of this research team, I am sure it won’t be long before they succeed in testing this possibility, and I look forward to finding out their results,” says Renée Baillargeon, a psychologist from the University of Illinois who studies theory of mind in infants.
Determining what another species is thinking is always challenging. Apes are hard to work with, which means that studies are usually plagued by small sample sizes. They can’t speak or be questioned directly, so scientists must probe their thoughts through elaborate experiments, which can almost always be interpreted in different ways. The question is, as the primatologist Frans de Waal posed in his recent book, “Are we smart enough to know how smart animals are?”
“Alternative explanations will always be available for any one experimental setup,” says Kristin Andrews, a philosopher at York University who studies animal cognition. But “as clever researchers find that apes perform well in new situations, they generate more and more evidence that apes understand others’ minds, and the piecemeal skeptical explanations start to look like unwarranted conservatism.”