Numbers and time run left to right for Westerners. Any direction will be enough for some Bolivians.
Consider a ruler, a calendar, or even weights at a gym lined up. Why do smaller values, earlier times, and lighter weights tend to be on the left, whereas larger or later values tend to be on the right?
Researchers have argued whether these mental number lines or the inclination to sort numerically from left to right, are intrinsic or taught since the early 1990s. In recent years, the argument has shifted from mental number lines to mental magnitude lines: the human proclivity to map any abstract concept in three-dimensional space, including numbers, time, and even facial emotions. Now, a study published in Science Advances on August 11 that compared largely adult Indigenous farmer-foragers in Bolivia to preschoolers and adults in the United States has landed squarely on the learning or culture side of the issue, adding new fuel to the fire.
"Numbers increase in one direction" for these Bolivians, known as the Tsimane people. Time moves in one direction only. Size grows in one direction only. But any direction will suffice," says the University of California, Berkeley cognitive scientist Benjamin Pitt. In other words, because they have minimal formal education to instruct them on which way to place numbers in space, the Tsimane people don't seem to mind whether heavier dumbbells are placed on the right or left.
Researchers believe that knowing how humans map abstract thoughts in space could reveal information about the development of spatial cognition. According to cognitive scientist Kensy Cooperrider, who recently completed his postdoctoral studies at the University of Chicago and is currently located in San Diego, these maps "provide a foundation upon which later mathematical and spatial abilities build."
Pitt and colleagues first asked members of three groups to arrange things on a horizontal board: 96 Tsimane teens and adults, 31 U.S. preschoolers, and 18 U.S. adults. Five index cards with one to five dots or five blocks varying in size from one to five inches were lined up by all of the participants. The team discovered that whereas all of the adults in the United States mapped the cards and blocks from smallest to largest on the right, the Tsimane adults and U.S. toddlers were equally likely to map in either manner.
The scientists then looked at how 60 Tsimane teenagers and adults mapped information along the x, y, and sagittal (front to back) axes. The researchers wanted to discover if the Tsimane people could map time to space in addition to asking them to order objects by size and quantity. The researchers instructed them to order five sets of bananas, ranging in hue or ripeness from extremely green to almost black, for the trial. For a total of nine tasks, each participant completed three mapping tasks per axis.
The Tsimane people, once again, showed no directional bias. On a given axis, a single participant frequently ordered one notion one way and another way, such as the lower magnitude green bananas on the bottom and the higher magnitude five-dot index card on the bottom. The researchers kept track of whether or not each participant placed all of the cards in the same direction on a given axis. The researchers discovered that mappings moved in the same direction just 42% of the time after averaging the scores across all participants.
"This work puts doubt on the assumption that we have an inbuilt system for spatializing numbers that many psychologists and cognitive neuroscientists have maintained," adds Cooperrider, who came to similar conclusions in a 2017 study evaluating mental number lines among the Yupno people of Papua New Guinea.
However, according to Emory University's cognitive scientist Stella Lourenco, the discussion is far from over. Participants in Tsimane did exhibit some consistency. For example, roughly 80% of the time, each participant organized the size and number cards in the same way along the x, y, and sagittal axes. "They examine the facts and conclude that there is inconsistency." "When I look at these numbers, I think to myself, 'It looks fairly good to me in terms of consistency,'" Lourenco adds.
According to a study on newborn babies, Lourenco believes that humans are born with natural mental maps, but that life experience obscures such default tendencies. Arabic speakers, for example, who read from right to left, place lesser magnitude things on the right, which is the opposite of native English speakers. "Directionality is flexible," she argues, even if you believe in default or natural directionality.
According to cognitive scientist Rosa Rugani of the University of Padua in Italy, the more pressing question is how humans construct mental maps. What kinds of experiences, for example, lead Tsimane people to map numbers and size cards in the same direction as number and time cards?
Furthermore, Rugani claims that an exclusive focus on directionality has disguised the most important subject of all: why do humans from preschoolers to the Yupno and Tsimane map abstract notions in space at all? "We need to go back to the beginnings of this conversation," she argues.