The Fat Face Illusion Tricks Humans but Not Chimpanzees

Author: Kate Baggaley


When it comes to optical illusions, humans aren't the only suckers. Animals fall prey to plenty of the same illusions that we do.

But sometimes, their judgment is more accurate than ours. Scientists have discovered one illusion that doesn't work on chimpanzees, even though it reliably misleads people. The way this illusion tricks us but fails to dupe our closest relatives might reveal how we differ in making sense of the world around us.

The illusion in question featured in a recent experiment conducted on chimpanzees and humans. Called the fat face illusion, it happens when we see an image of two identical faces, one hovering above the other. The face on the bottom appears to be fatter than the one on top.

Fat face illusion. Source: i-Perception

This trick closely resembles the Jastrow illusion, in which two arcs are stacked one above the other. The top arc seems smaller, though in reality they're identical. It seems that we misjudge the size of the top arc because we're comparing its lower right corner with the bottom arc's upper right corner.

Image: Fibonacci via Wikimedia Commons, CC BY-SA 3.0

Masaki Tomonaga, a researcher at Kyoto University, wanted to find out how similar these illusion really are. He and his team presented them both to humans and chimpanzees at the Primate Research Institute in Aichi, Japan.

The chimps and people had to view faces and shapes on a computer screen, one above the other, and click on the thinner or narrower one of each pair. Many pairs really did have objects of different sizes. But in some cases, the arcs or faces were actually identical.

When shown twin arcs, both species made the same mistake. They were more likely to think the top arc was smaller than the bottom one, showing that chimpanzees and people are both susceptible to the Jastrow illusion.

But when it came to judging faces, a difference emerged between humans and our simian relatives. When viewing identical human or chimpanzee visages, people tended to think the bottom face was fatter. As expected, they'd been fooled by the fat face illusion. The chimps, on the other hand, had no bias for the top or bottom face, indicating that the illusion holds no sway over them.

Image: Masaki Tomonaga (2015).

The chimps' ability to see one illusion but not the other suggest that the fat face illusion could be a distinct phenomenon and not just a case of the Jastrow illusion.

For humans to see the fat face illusion, we need to recognize that we are looking at a face in the first place. Scientists discovered in an earlier experiment that if we've got a face-like outline to guide us, we can perceive the illusion even in line drawings of faces, faces with scrambled features and empty faces with no eyes, noses or mouth.

Chimps may rely less on outer contours to recognize a face than people do, noted Tomonaga, who published the findings December 14 in the journal i-Perception. This could explain why the fat face illusion doesn't work on them.

We know that chimpanzees have complex social lives, and that it's important for them to be able to recognize faces, as in people. Scientists have seen that faces quickly capture chimpanzees' attention. But chimps' brains might not approach faces quite the same way as ours.

"The results of the present experiment may suggest that the basic-level processing of the face in chimpanzees is different from humans," Tomonaga concluded.