Knowing a language that uses counting words can shape one’s ability to understand large numbers.

A new study of deaf people who have made up their own hand signals to communicate shows that without number words, it’s hard to keep track of more than three objects at a time.

“Learning language really shapes the way we think,” said cognitive psychologist Elizabet Spaepen of the University of Chicago, lead author of a paper published Feb. 7 in the Proceedings of the National Academy of Sciences. “It can change the way we conceptualize something as seemingly basic as number.”

Psychologists had...