What’s this guy thinking? Does he know what I know? Most of us develop the ability to make inferences about what other people might be thinking, the hallmark of “theory of mind,” at age four. Scientists have long known that the acquisition of language plays a role in this process, but so far it had been unclear whether social experience could substitute for it. A new study suggests it cannot.

Jennie Pyers of Wellesley College and her colleagues studied deaf adults in Nicaragua. Some of the participants had learned an early, rudimentary form of Nicaraguan sign language (NSL), whereas others were fluent in a more sophisticated form of NSL that included mental state terms, such as “know” and “think.” Pyers and her team had all signers undergo a so-called false-belief test in which signers looked at a sequence of pictures showing two boys playing in a room and storing a toy underneath a bed. After one of the boys leaves the room, the other moves the toy to a different location. Study participants then had to choose between two pictures to complete the series: the first showed the returning boy looking for the toy in its original location on reentering the room, and the second showed him looking in its new location.

Those Nicaraguans with complex sign language skills were more likely to choose the first picture—indicating an understanding of false belief—than were those with less developed language skills. Moreover, after a two-year period during which early signers improved their NSL knowledge, they performed better at the false-belief task.

The findings support the hypothesis that although an implicit understanding of other people’s knowledge and belief states develops early in life, advanced language is needed “to unlock the ability to productively use it,” Pyers says.

Note: This story was originally published with the title "Reading Minds"