monkey magic trick

Illusion involving a hidden thumb confounds capuchin and squirrel monkeys for the same reason as humans – it misdirects the expected outcomes of actions they can carry out. (Credit: University of Cambridge)

CAMBRIDGE, United Kingdom — Monkeys are smarter than humans — at least when it comes to magic tricks. In a new study, psychologists show that there’s no fooling most primates when they see the sleight-of-hand trick. The reason? They lack opposable thumbs.

The “French drop” is a sleight-of-hand trick where an object seems to vanish into thin air. In reality, the small object is discreetly taken from one hand by the hidden thumb of the other hand. The monkeys that lacked opposable thumbs saw through the deception, while those possessing the extra digit were left wondering where their treat went.

Does this mean some monkeys are just wiser than their comrades? Researchers say yes and no. It appears that monkeys who have opposable thumbs were more easily duped, but it’s because their own experiences with using thumbs make them assume others with the same anatomy perform similar movements.

Magicians use intricate techniques to mislead the observer into experiencing the impossible. It is a great way to study blind spots in attention and perception,” says Elias Garcia-Pelegrin, an assistant professor at the National University of Singapore who has practiced magic for a decade and conducted the study during his PhD at Cambridge, in a university release.

French drop magic trick
(Credit: University of Cambridge)

Understanding how human’s closest relatives react to magic can give more insight into how our minds evolved in a way that magicians can easily fool us.

“In this case, whether having the manual capability to produce an action, such as holding an item between finger and thumb, is necessary for predicting the effects of that action in others,” says Garcia-Pelegrin.

The French drop is one of the first tricks a rookie magician learns

The trick starts with a coin shown in one hand. The other hand then reaches over and grabs it. The palm of the second hand faces inwards, with the magician’s thumb concealed behind their fingers. The audience them assumes the coin was taken by the second hand — only for the magician to reveal that palm is empty. The reveal is that the magician secretly dropped the coin into the palm of the original hand.

Monkeys don’t care much for coins so the psychologists in the current study used food instead to capture their attention. If they correctly guessed which hand held the food, they would receive it as a reward. The scientists predicted that monkeys with opposable thumbs would act like human audiences and assume the hidden thumb would grab the item and choose the wrong hand.

Twenty-four monkeys saw the French drop. Eight capuchins paid close attention to peanuts, eight squirrel monkeys kept their eyes on dried mealworms, and eight marmosets watched marshmallows. Like humans, capuchins have opposable thumbs and have put them to good use. Compared to other primates, they do better at using stone tools to crack nuts in the wild and waggle each finger. Their opposable thumbs also give them a precision grip between the thumb and forefingers. As expected, most capuchins were fooled by the French drop. Only 19 percent correctly guessed the right hand and were able to get their coveted peanuts.

Squirrel monkeys also have opposable thumbs but are far less skillful with their hands than capuchins and have limited thumb rotation. Despite having worse dexterity, the monkeys still anticipated the magician’s thumb to take the treat, getting fooled 93 percent of the time.

“This suggests that a monkey doesn’t have to be expert in a movement in order to predict it, just roughly able to do it,” explains Garcia-Pelegrin.

There’s no tricking a clever marmoset

Marmosets, on the other hand, do not have opposable thumbs. Their thumb aligns with their fingers to make five equidistant digits, giving them a hand grip that’s perfect for climbing thick tree trunks. Marmosets were rarely fooled by magic and only guessed the wrong hand six percent of the time. They chose the hand where the marshmallow was first placed and stuck with it.

The case for thumbs or no thumbs gets stronger when looking at creatures with no hands at all. Previous research found that birds like the Eurasian jay make similar choices as marmosets when seeing a French drop.

When the magicians actually completed the hand-to-hand transfer instead of misdirecting the animals, the capuchins and squirrel monkeys correctly guessed the hand while the marmosets did not. Tweaking the trick a bit further, the psychologists created their own version of the trick called the “Power drop.” This uses a full fist grab — a hand action all monkeys can perform. The power drop fooled all the monkeys most of the time.

“There is increasing evidence that the same parts of the nervous system used when we perform an action are also activated when we watch that action performed by others,” says Nicola Clayton, a professor at Cambridge’s department of psychology and senior study author. “This mirroring in our neural motor system might explain why the French drop worked for the capuchins and squirrel monkeys but not for marmosets.”

The study is published in Current Biology.

About Jocelyn Solis-Moreira

Jocelyn is a New York-based science journalist whose work has appeared in Discover Magazine, Health, and Live Science, among other publications. She holds a Master's of Science in Psychology with a concentration in behavioral neuroscience and a Bachelor's of Science in integrative neuroscience from Binghamton University. Jocelyn has reported on several medical and science topics ranging from coronavirus news to the latest findings in women's health.

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1 Comment

  1. Sherry Munson says:

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