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Can you fool a monkey with a magic trick? Only if it has opposable thumbs – Ars Technica -Se

A Humboldt’s squirrel monkey is fooled by the “French drop” magic trick. Credit: E. Garcia-Pellegrin et al., 2023

The key to a successful sleight-of-hand magic trick is how well a magician manipulates the audience’s perception, especially of manual movements, since this is crucial to how we anticipate the actions of others. To learn more about how humans experience such misdirection, UK researchers performed simple magic tricks on three species of monkeys to see if they could be fooled. They found that only those species with at least partially opposable thumbs were fooled, suggesting that similar anatomy (and therefore biomechanical ability) plays an important role in confusion. They described their results A new paper Published in the journal Current Biology.

“Magicians use complex tricks to confuse the observer into experiencing the impossible,” said co-author Elias Garcia-Pellegrin, who practices magic and conducted this research while completing his PhD at Cambridge University. “This is a great way to study blind spots in attention and perception. By investigating how primate species experience magic, we can understand more about the evolutionary roots of the cognitive errors that expose us to magicians’ cunning. In this case, Kina holds an item between finger and thumb. Having the manual ability to create an action, such as putting, is necessary to predict the effect of that action on others.”

The researchers focused on three species with different hand anatomy and associated biomechanical abilities: yellow-breasted capuchin monkeys, Humboldt’s squirrel monkeys, and common marmosets. For example, capuchins are known for their manual dexterity, as they can control their fingers individually. So they can perform a scissor grip (holding an object between the sides of two fingers), as well as a precision grip (bringing the finger to the index or middle finger). They can even probe, pinch, or grasp an object with both hands, like humans, and use stone tools to crack nuts.

Squirrel monkeys are not nearly as dexterous by comparison, but they have been known to occasionally use simple tools. They have hinge-like joints that limit thumb rotation, so the thumbs are not completely opposable. But they can still touch the index side of the middle finger (though not the pad). Marmosets, on the other hand, evolved for vertical movements like climbing tree trunks, and opposable thumbs wouldn’t be an advantage for that, so they don’t have them. They have rigid thumbs instead. According to the authors, marmosets climb by extending their five digits as wide as possible to increase the surface area, simultaneously flexing all digits to dig with their claws. They use a combination of power grips and scissor grips to manipulate objects.

to enlarge / Three species of New World monkey participated in the experiment, each with inherently different hand anatomy and biomechanical capabilities.

E. Garcia-Pellegrin et al., 2023

The researchers decided to use one of the most basic sleight of hand tricks in the magician’s playbook for their experiment: the “French drop.” The magician then holds a coin in one hand, then reaches over it with the other hand and grasps it with the palm facing up, thus hiding the coin behind the fingers. The idea is for the audience to focus on the second hand and assume that the coin has moved. But when the magician opens that second hand, it is empty—because the magician actually dropped the coin into the original palm. The ability to block the thumb is the key to magic tricks, so the thumb must be opposable.

García-Pellegren and colleagues offered pieces of food instead of coins: peanuts for capuchins, dried insects for squirrel monkeys, and marshmallows for marmosets. One version of the experiment involved all the monkeys performing French drops to see which ones had been fooled. They also ran a control experiment where pieces of food were actually transferred from one hand to the other. Finally, the team conducted a third version of the experiment using a modified magic technique they called the “power drop,” using a full fist grip—a hand action that all three monkey species could perform. If the monkeys correctly guessed which hand held the hole, they could eat the chickpea as a reward.

Capuchin, as predicted, was fooled by both the French drop and the modified power drop technique. They chose the wrong hand on those trials about 81 percent of the time. But they chose correctly in the direct transfer test. “This pattern of preference has also been observed in people who are (also) generally confused by the French drop magic effect, but not by its actual transfer,” the authors wrote.

The movements required to perform the French drop, power drop and their real transfer counterparts
to enlarge / The movements required to perform the French drop, power drop and their real transfer counterparts

E. Garcia-Pellegrin et al., 2023

Squirrel monkeys, with their partially opposable thumbs, were also fooled by the French drop trick, choosing the wrong hand 93 percent of the time, contrary to what the researchers expected. “Squirrel monkeys couldn’t do a perfectly accurate grip, but they were still stupid,” said Garcia-Pellegrin. “This suggests that a monkey doesn’t have to be an expert at a movement to predict it, just being able to do it roughly.”

But marmosets had the opposite pattern. They chose correctly for the French drop effect most of the time, were fooled only 6 percent of the time, but chose incorrectly when the morsel was transferred to the other hand. This suggests that they perceived hand finesse differently than the other two types of monkeys, possibly because they lacked opposable thumbs. “It appears that in this case, the marmosets likely used a simple heuristic to choose the hand that primarily contained the reward regardless of the pantomime action performed by the experimenter,” the authors wrote — a similar choice mechanism used by corvids, which have no thumbs at all.

According to the authors, their results indicate that perception—including the ability to predict the manual movements of others—is greatly influenced by one’s inherent physical abilities.

“There is growing evidence that the same parts of the nervous system that are used when we perform an action are also activated when we see that action performed by others,” said co-author Nicola Clayton, a psychologist at Cambridge University. “This mirroring in our neural motor system may explain why the French drop worked for capuchins and squirrel monkeys but not for marmosets. It’s about the embodiment of knowledge. How one’s fingers and thumbs move can inform our thinking and the inferences we make about the world – as well as help shape what others see, remember and expect based on their expectations.”

DOI: Current Biology, 2023.10.1016/j.cub.2023.03.023 (Regarding DOI).



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