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Learned, not innate human intuition: Study finds twist to the story of the number line

Tape measures. Rulers. Graphs. The gas gauge in your car, and the icon on your favorite digital device showing battery power. The number line and its cousins – notations that map numbers onto space and often represent magnitude – are everywhere. Most adults in industrialized societies are so fluent at using the concept, we hardly think about it. We don’t stop to wonder: Is it “natural”? Is it cultural?

“Influential scholars have advanced the thesis that many of the building blocks of mathematics are ‘hard-wired’ in the human mind through millions of years of evolution. And a number of different sources of evidence do suggest that humans naturally associate numbers with space,” said Nunez, coauthor of “Where Mathematics Comes From” and co-director of the newly established Fields Cognitive Science Network at the Fields Institute for Research in Mathematical Sciences.
“Our study shows, for the first time, that the number-line concept is not a ‘universal intuition’ but a particular cultural tool that requires training and education to master,” Nunez said. “Also, we document that precise number concepts can exist independently of linear or other metric-driven spatial representations.”

Nunez and colleagues asked Yupno adults of the village of Gua to complete a task that has been used widely by researchers interested in basic mathematical intuitions and where they come from. In the original task, people are shown a line and are asked to place numbers onto the line according to their size, with “1” going on the left endpoint and “10” (or sometimes “100” or “1000”) going on the right endpoint.

The researchers found that unschooled Yupno adults placed numbers on the line (or mapped numbers onto space), but they did it in a categorical manner, using systematically only the endpoints: putting small numbers on the left endpoint and the mid-size and large numbers on the right, ignoring the extension of the line — an essential component of the number-line concept. Schooled Yupno adults used the line’s extension but not quite as evenly as adults in California.

The researchers ran several experiments while in Gua, Papua New Guinea, including those that examine another fundamental concept: time. In their time study with the Yupno, now in press at the journal Cognition, Nunez and colleagues find that the Yupno don’t use their bodies as reference points for time – but rather their valley’s slope and terrain. Analysis of their gestures suggests they co-locate the present with themselves, as do all previously studied groups. (Picture for a moment how you probably point down at the ground when you talk about “now.”) But, regardless of which way they are facing at the moment, the Yupno point uphill when talking about the future and downhill when talking about the past.
Interestingly and also very unusually, Nunez said, the Yupno seem to think of past and future not as being arranged on a line, such as the familiar “time line” we have in many Western cultures, but as having a three-dimensional bent shape that reflects the valley’s terrain.