Only Monkeys With Opposable Thumbs Fell for This Classic Magic Trick

A team of researchers performed magic tricks on several species of monkeys, and they discovered that the animals’ cleverness may depend on the structure of their hands.

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Specifically, the team found that the monkeys were more likely to be fooled by the sleight of hand if they had opposable thumbs. The research is published today in Cell Biology.

Magic tricks are a great way to test the intelligence, perception and cognition of animals. The researchers used a classic trick called the “French drop” to test the recognition of three types of monkeys: yellow-breasted capuchins, squirrel monkeys and common marmosets. The former two species have opposable thumbs, while marmosets do not.

The magic trick requires an opposable thumb, and the idea here was to test “if having the manual ability to produce an action, such as holding an item between your finger and thumb, is necessary to predict the effects of that action to others”, he said. Elias Garcia-Pelegrin, a cognitive scientist at the National University of Singapore, in a University of Cambridge publication.

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After several deception performances, the research team found that the primates’ morphology determined whether they were deceived, fooled, lost, confused, and tricked by the monkeys.

French is an old trick, one you may remember from birthday parties as a child. The performer shows the audience a small object, such as a coin, held with the back of the hand facing the crowd, with the fingers pressed together and pointing to the sky. The performer then uses the other hand to hide the object and imitates picking it up, but really lets the object fall into the palm of the original hand. The audience is then usually asked to figure out where the object is.

The illusion leads people to believe that the object has changed hands when it has not. The key to the trick is to hide the thumb of the second hand, hidden behind the other fingers.

For the final experiment, the coin was replaced with a candy: peanuts for the capuchins, mealworms for the squirrel monkeys, and marshmallows for the marmosets. Capuchins and squirrel monkeys largely fell for the trick (81% and 93% of the time, respectively), but marmosets didn’t—they were only caught 6% of the time.

“This reflection in our motor nervous system may explain why the French drop worked for capuchins and squirrel monkeys, but not for marmosets,” said Nicola Clayton, a cognitive scientist at the University of Cambridge, in the same publication.

To confirm that the marmosets weren’t just more sensitive to the switch (perhaps they can smell pleasure while other species can’t!), the researchers also performed a modified version of the French drop. This maneuver, called the “power drop,” was the same basic trick, but the treats were transferred using a grip that all three species were capable of.

Power drops fooled capuchins 81% of the time and squirrel monkeys and marmosets 94% of the time, suggesting that the marmosets’ ability to follow the deception was due to the positioning of the magicians’ digits.

“It’s about embodying knowledge,” Clayton added. “The way one’s fingers and thumbs move helps shape the way we think and the assumptions we make about the world—as well as what others can see, remember, and predict, based on their expectations.”

Scientists created monkeys from monkeys with a simple trick. But don’t feel too bad for the animals: when they correctly guessed the location of the sweets, they got to eat them.

More: Why scientists should use magic to study animal intelligence