Magic trick only fools monkeys with opposite thumbs, study suggests

Scientists performed a famous magic trick for three species of monkeys – capuchins, squirrel monkeys and marmosets – with different hand structures.

They discovered that to deceive, a conjurer needs the same anatomy as his audience.

The psychologists used a sleight of hand trick called the French drop, in which an object appears to disappear when a bystander assumes they are grasping it in one hand with the hidden thumb of the other hand.

The French drop is one of the first tricks any budding magician sets out to master.

A coin is shown in one hand, and the other hand reaches out and grabs it. The palm of the second hand faces inward, with the magician’s thumb tucked behind the fingers.

The audience knows that the thumb is lurking, ready to grab, so they assume the coin has been taken when it is no longer visible. Her attention follows the second hand, only to find it empty at the reveal.

The magician secretly drops the coin into the palm of the original hand.

The study, carried out at the University of Cambridge’s Comparative Cognition Laboratory, found that monkeys lacking opposable thumbs did not fall for the trap and kept informed about the whereabouts of the delicacies a magician was trying to make disappear.

According to the researchers, sharing a biomechanical skill may be necessary to accurately anticipate the movements of those same limbs in other individuals.

Dr Elias García-Pelegrín, who has practiced magic for a decade and carried out the experimental work during his PhD at Cambridge, said: “Magicians use intricate techniques to fool the observer into experiencing the impossible. It’s 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 cognitive deficiencies that leave us open to the cunning of magicians.

“In this case, it is necessary to have the manual ability to produce an action, such as holding an object between the thumb and index finger, to predict the effects of that action on others.”

In the experiment, morsels of food replaced coins for the monkeys and were given out as rewards, but only if the animals guessed the correct hand.

The trick was performed repeatedly on 24 monkeys: eight capuchins were dazzled with peanuts, eight squirrel monkeys with dried worms, and eight marmosets with marshmallows.

Capuchins, who have opposable thumbs and are famous for their dexterity (they use stone tools to crack nuts in the wild), were regularly fooled by the trick (81% of the time).

Squirrel monkeys are less dexterous than capuchins, with limited thumb rotation, but they can oppose their thumbs.

They cannot perform a precision grasp in the same way as capuchins and humans, experts say.

However, squirrel monkeys were routinely fooled by disappearing mealworms (93% of the time).

“Squirrel monkeys can’t make grasps with full precision, but they were still fooled. This suggests that a monkey does not have to be an expert in a movement to be able to predict it, but simply capable of doing it,” said Dr. García-Pelegrín, recently appointed assistant professor at the National University of Singapore.

Marmosets do not have opposable thumbs and were rarely caught by magic (just 6% of the time), the study found.

They just chose the hand the marshmallow was initially placed in and stuck with it.

The team also attempted to nullify the tricks by completing one-on-one transfers, rather than deflect with a French drop.

This time, the capuchins and squirrel monkeys correctly anticipated and dined out, and the marmosets missed out.

Eventually, the scientists came up with their own version of the French drop, which they call the “power drop.”

It uses a manual action that all monkey species can perform, essentially a full fist grab. The power drop fooled all monkey species the vast majority of the time.

The study is published today in the journal Current Biology.


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