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By the time most kids get to kindergarten, they can at least count to ten. But some people use a communication system that does not offer a way to count even to five. A new study looks at how that changes the way they understand big numbers. NPR's Jon Hamilton reports.

JON HAMILTON: The study involved several adults in Nicaragua who were born deaf and never learned Spanish or a formal sign language.

Ms. ELIZABET SPAEPEN (Researcher, University of Chicago): So they essentially come up with a gesture system to communicate with the people around them.

HAMILTON: Elizabet Spaepen is a researcher at the University of Chicago. She says the gestures let them indicate a little as opposed to a lot, but not exact numbers.

Ms. SPAEPEN: Up to three they're fine. But then past three when we would resort to our counting system, they start to fall apart and they start to be only approximate.

HAMILTON: Spaepen and her colleagues learned that through a series of experiments. For example, she would knock her fist against theirs a certain number of times, then ask them to respond with the same number of knocks.

Ms. SPAEPEN: So if I were to knock four times on their fist, they might knock my fist five times.

HAMILTON: They couldn't tell the difference between four of something and five of something. Even though these are people who hold jobs, who use money, and are surrounded by friends and family who use exact numbers all the time.

It was a clue about just how much language affects our understanding of numbers. That's been a big question since back in 2004, when other scientists published research on two tribes in the Amazon. Spaepen says these tribes also lack precise words for big numbers.

Ms. SPAEPEN: What they have are words that sort of, kind of mean one and two but they're not even really exact. And then they have a word to mean many.

HAMILTON: And they, too, had trouble matching an exact number of knocks.

Unidentified Man: (Foreign language spoken)

Unidentified Woman: (Foreign language spoken)

HAMILTON: This videotaped experiment involved a member of the Piraha tribe.

(Soundbite of knocking)

Unidentified Woman: (Foreign language spoken)

HAMILTON: The researcher knocks four times. The Piraha tribe member only three. But Spaepen says some scholars felt these earlier studies failed to prove that language was the problem.

Ms. SPAEPEN: The argument has always been with those tribes that they also don't live in moneyed economies. So it may not be that they don't have the concepts. It may be that they just never have any pressure to use the concepts that they do have.

HAMILTON: One of the scholars making that argument was Daniel Casasanto, a researcher at the Max Planck Institute for Psycholinguistics in the Netherlands. But Casasanto says he's not so skeptical anymore.

Mr. DANIEL CASASANTO (Max Planck Institute for Psycholinguistics): What's exciting about this new study is that these people are immersed in a culture that has large exact number and in which exact enumeration is very important.

HAMILTON: Casasanto says the study suggests how language augments our natural ability to understand numbers. He says all people, and some animals, have brains that can tell the difference between, say, two cookies and three cookies on a plate.

The human brain is also very good at assessing approximate values, like the difference between 10 and 20 cookies. But Casasanto says the brain needs some sort of counting system to tell the difference between 10 cookies and 11.

Mr. CASASANTO: What language does is give you a means of linking up our small, exact number abilities with our large approximate number abilities.

HAMILTON: And for people in developed countries, he says, that's essential.

Mr. CASASANTO: It has been the tool that gave rise to the society we live in. The skyscrapers we work in and the computers that we're talking on right now and all of these things are possible because of large exact number and our ability to manipulate them.

HAMILTON: The new research appears in the Proceedings of the National Academy of Sciences.

Jon Hamilton, NPR News.

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