Newborn babies seem to have a rudimentary sense of numbers, preferring smaller numbers on the left and larger ones on the right. The finding suggests that this left-to-right mental number line might be innate for humans.

We visualize most of our thoughts in space. “Anything you want to remember that has a sense of order – be it days of the week or musical tones—you tend to map that to a spatial continuum,” says Koleen McCrink at Barnard College, New York.

The same is true for numbers. In Western cultures, people tend to think of numbers increasing in value along a mental number line from left to right, while people who speak Arabic and Hebrew picture numbers running in the opposite direction.

To see if number lines are innate, or determined by language and culture, Rosa Rugani at the University of Padua, Italy, and her colleagues looked for mental number lines in newborn babies between 12 and 117 hours old. The average age of the babies was just 55 hours.

Rugani’s team showed each of the babies a series of images in which white squares contained a number of smaller black squares. Half the time, the babies were shown two white squares each containing four black squares, side by side. The rest of the time, the babies were shown two white squares that contained 36 black squares.

Early preference

An eye-tracker device that monitored where the babies were looking revealed that babies looked towards the left more when shown the smaller number of black squares, and towards the right more when shown the larger number of black squares.

“Seeing this left-to-right mental number line in newborns is a powerful suggestion that it is innate,” says McCrink, who was not involved in the research. She thinks our innate sense of order might run from left to right because the right side of the brain, which receives visual information from the left eye, is larger in babies. “Maybe that’s why we pay more attention to the left side first,” she says.

“It’s pretty impressive to have data from a newborn, where there is very little possibility for cultural influence,” says Martin Fischer at Potsdam University in Germany. However, that doesn’t mean culture hadn’t already had an effect in these babies. “Just spending five minutes with an adult could influence children,” says Fischer.

All the babies in the study were born in Italy, so it’s possible that an experiment in an Israeli hospital might produce different results. In addition, babies born in Arabic countries whose reading is from right to left would be interesting to study using this model.

Samuel Shaki at Ariel University, in Israel thinks the findings may tell us something about how newborns process complexity. Instead of having an innate number line, it could be that the babies in the study associate simpler images with the left and more complex ones with the right, he says.

If further evidence is found of an innate mental number line, the next step will be to figure out what it means for our cognition and maths skills, says McCrink. Previous research has found that 6-month-old babies who prefer to watch sequences of changing number values over repeating ones are more likely to have a better grasp of early maths skills three years later, for instance.