Sleight-of-hand magic trick only fools monkeys with opposable thumbs (April 4, 2023)

Manual action expectation depends on biomechanical ability across primates, using a magic misdirection paradigm. Researchers at the University of Cambridge tested a sleight-of-hand “French drop” trick with three New World monkey species that differ in digit structure: capuchins with opposable thumbs, squirrel monkeys with limited thumb rotation, and marmosets with non-opposable digits. Capuchins and squirrel monkeys were frequently fooled about which hand held the food reward, consistent with shared motor capabilities shaping what each animal expects to happen. Marmosets were rarely fooled and instead chose the hand based on initial placement. A control condition where scientists actually performed the transfers, plus a more universal “power drop” using a fist-grab action, further supported the idea that an individual’s inherent physical capability influences perception, memory of what was seen, and predictions of others’ manual movements.