Rock smashes scissors. Scissors cut paper. Paper covers rock. The rules behind the favorite game of schoolyard kids and adults deciding who takes out the trash are pretty simple. But they also represent a kind of logic problem. Four-year-olds can learn the rules, and so can chimpanzees—but the differences in how kids and apes become proficient reveal a little about how their minds work.
The relationship between the three items in rock-paper-scissors is circular. There isn't a straightforward hierarchy among them. Animals in the wild can benefit from understanding linear hierarchies, such as the order of dominance in a group, or who can fit inside whose mouth. But earlier studies have shown that some animals, such as monkeys, rats and pigeons, can also learn circular relationships.
At the Primate Research Institute at Kyoto University in Japan, scientists attempted to teach seven adult chimpanzees the basics of rock-paper-scissors. In daily training ...
