Imagine taking out a 12-inch ruler and finding that the number 12 is on the left side and the number 1 is on the right side. For most native English speakers, this would be disorienting. We are used to seeing the numbers move from smallest to largest, from left to right. When this layout flips, people struggle because the numbers are now in the "wrong" place.
Psychologists have long known that people in Western cultures tend to associate smaller numbers with the left side of space and larger numbers with the right, a phenomenon called the SNARC effect - short for Spatial-Numerical Association of Response Codes.
In the lab, researchers like us test this tendency by asking people to press a left or a right button when shown a numerical digit. Native English speakers are generally quicker to press left for small numbers and right for large numbers because these locations match our mental number line.
But here's the twist: What feels like the "correct" direction depends on where you grew up and where you live. In places with right-to-left languages like Arabic, the pattern often flips : People are faster to press right for small numbers and left for large numbers. Speakers of Farsi, a right-to-left language, who were born in Iran but move to France gradually shift toward a left-to-right mapping the longer they stay .
Even literacy matters . On average, people who never learned to read or count don't show the effect at all. Researchers aren't sure why. Maybe these people do not map numbers to space. Or maybe each individual has their own different orientation - left-to-right vs. right-to-left - that wash each other out when investigators average them all together.
Although people in Western cultures are used to seeing numbers increase left to right on keypads, rulers or classroom number lines, the SNARC effect isn't limited to numbers. In the lab, similar left-to-right patterns appear with other magnitudes, including size , height and brightness .
A key question is the origin of the SNARC effect. Some researchers point to brain lateralization : the differences in how the left and right sides of the brain are wired and used. Others suggest it is a broader cognitive habit : When people line things up, they prefer to sort them in an order that makes sense for them. For example, if you are comparing 5 inches to 9 inches, you might think of 5 on the left and 9 on the right. But if you were comparing 5 o'clock to 9 o'clock, you might think of 5 on the right and 9 on the left, based on the face of an analog clock.
But culture matters, too : Cultural experience learning that "small" is on the left and "large" is on the right results in a stronger SNARC effect. It's therefore not yet clear where the SNARC effect comes from because in humans, biology and culture are all tangled up.
Do other animals have mental number lines?
Our field of study is comparative cognition. We study how primates and birds make sense of the world: how they think, learn and remember. Animals share many cognitive processes with humans, but lack cultural experiences like reading, writing and counting, making them ideal subjects for investigating this number-line question.
We and other researchers in our field started by developing a SNARC task for nonhuman animals. We showed orangutans and gorillas two sets of dots on a touchscreen, one on the left and the other on the right. If these animals naturally associate "less" with left and "more" with right, then on average they should have been more accurate and faster at picking out the smaller set when it appeared on the left than when it appeared on the right. But that is not what happened.
Looking closer at the individuals, we saw why: Some apes showed a left-to-right pattern and others preferred right-to-left. These individual preferences canceled each other out in our overall averaged results. This split suggested that apes, like humans, do organize magnitudes in space. But without cultural cues like reading or counting direction, each animal developed its own preferred ordering direction.
We and others have since replicated the original study in rhesus monkeys , pigeons and blue jays and our ongoing, not yet peer-reviewed study with chickens. In all of these cases, there's strong evidence for spatial representation of magnitude, along with clear individual differences in direction.
Number-line direction may not be so clear-cut
Finding so much variability in animals made us think: Might individual people also differ more than the averages suggest? Many SNARC studies report only average scores combining all the people tested, making it hard to see whether individual people vary like other animals do.
So we ran a new study in which native English speakers from the United States judged different magnitudes ranging from Arabic numerals to dot quantities and the brightness of a square. The averages showed the expected left-to-right pattern. But individuals often didn't .
Nearly a quarter of participants judging dot quantities showed a right-to-left pattern, contradicting their reading and counting history. When judging brightness of a square, the split was almost 50/50, erasing the average effect altogether, just like in animals.
Our results suggest that the SNARC effect isn't a universal rule etched into human brains by culture. Instead, it looks more like a flexible way of thinking that can vary among individuals, species - or even from task to task in the same person. Some people like arranging things left-to-right, others prefer right-to-left, and the same is true of animals.
By looking beyond averages, we see a richer story: Minds can be flexible and inventive, whether they belong to apes, birds or humans.
The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
