The SNARC effect is a fascinating phenomenon (and no, it has nothing to do with cheeky one-off blog posts). When asked to recognize numbers, people react faster with their left hand for low numbers, and faster with their right hand for high numbers. Take a look at this graph:
This shows the results of an experiment led by Samuel Shaki: Twelve Canadian university students were shown a series of single-digit numbers. Their task was simple: as quickly as possible, press one button if the number is odd, and another button if the number is even. This graph charts reaction time of the right hand minus the reaction time of the left hand (the odd-even buttons were switched halfway through the experiment, so each hand had an equal number of tries with each number).
It takes a little explanation to understand what you’re seeing here. For the number “1”, the difference in reaction time is a little over 50 milliseconds, meaning the right hand is slower than the left hand. For “9” the difference is a little more than negative 60 milliseconds, meaning the right is faster than the left hand. The pattern across the span of the digits is similar: respondents are better with their left hand for low numbers, and better with the right hand for high numbers.
This result has been replicated in dozens of studies, one of which we’ve discussed on Cognitive Daily. What’s less clear is exactly why the result occurs. It’s possible that because we’re accustomed to seeing number-lines with lower values on the left (like on the graph above), we respond more quickly with the left hand to low values. But it’s also possible that left-to-right reading of words is what leads to the effect. In fact, Japanese readers (who typically read top-to-bottom) have been tested with a vertical arrangement of buttons, and they respond faster to lower numbers with the top button.
So Shaki’s team decided to test 11 Palestinian students who read only Arabic text and Arabic-Indic digits, both of which are read from right-to-left instead of left-to-right. Here are the results:
For Palestinians, there’s still a SNARC effect, but it’s reversed from the Canadian SNARC effect: They’re faster with the right hand for smaller numbers, and faster with the left hand for larger numbers. That’s powerful evidence that there’s no left-to-right number line in everyone’s head; the effect seems to depend on how you learned to read words and numbers.
But Shaki’s team wasn’t finished. Most Israeli students learn to read Hebrew text (right-to-left), but also learn Arabic numbers, which are read left-to-right. Here are the results for 16 Israeli students on the same test:
Now there’s no SNARC effect at all — the results show no significant difference in response time for the right or left hand for any portion of the number range. Even when the researchers split the Israeli students into groups with slightly positive and slightly negative slopes, they couldn’t find an orderly effect in either group, so it doesn’t appear to be the case that some Israelis have a left-right SNARC effect while others have a right-left effect.
The researchers say this probably means that the SNARC effect is a result of the combination of left-right (or right-left) reading and left-right/right-left numbering — but there may be other contributors, such as preferred order for finger-counting or for counting real objects.
Shaki, S., Fischer, M., & Petrusic, W. (2009). Reading habits for both words and numbers contribute to the SNARC effect Psychonomic Bulletin & Review, 16 (2), 328-331 DOI: 10.3758/PBR.16.2.328