A new study has found some secrets of people's understanding of large numbers. Researchers studied a group of people who were born deaf and never learned any spoken language or a formal sign language, but they have developed a gesture system to communicate with people around them. The gestures let them express approximate amounts, but not exact numbers. "Up to three, they're fine," says Elizabet Spaepen, a researcher at the University of Chicago and an author of the study. "But past three, they start to fall apart." In one test, Spaepen would knock her fist against a study participant's fist a certain number of times and then ask them to respond with the same number of knocks. "If I were to knock four times on their fist, they might knock on my fist five times," she says. The finding offers a clue to just how much language affects our understanding of numbers. That has been a big question since 2004, when other researchers published data on two tribes in the Amazon whose members also lack words for big numbers. "What they have are words that mean one and two," Spaepen says, "and then they have a word to mean many." Members of the Amazonian tribes also had trouble matching numbers larger than three or four. But some scholars felt that these earlier studies failed to prove that language was the reason. They pointed out that the tribes lived in groups that didn't use money and had no need for exact numbers. The new research appears to answer that criticism. "It proves that the kinds of problems in understanding numbers that we found in the Amazonian tribes are not due to just the cultural or environmental circumstances," says Peter Gordon of Columbia University, one of the researchers involved in the earlier studies. The participants of the new study   _  .
cannot communicate with one another use a formal sign language to express numbers have some physical disability come from a distant tribe