‘Mental number line’ refers to a tendency to associate small numbers with our left-space and large numbers with our right-space. The first description of number-space mapping goes back to 1880, when Sir Francis Galton first reported that humans, in many cases, describe and think of numbers as being represented along a line. Galton called this line the “mental number line” oriented from left to right. More than one hundred years later, a group of French scientists provided the first experimental demonstration of the mental number line in adult humans.

For a long time, the orientation of this mental number line has been attributed to cultural factors, such as reading and writing habits. In recent decades, a growing number of studies demonstrated that infants, newborns and animals associate numbers and space. This evidence suggests that culture plays a secondary role in determining the orientation of the mental number line.

Selected references

A mental number line in human newborns.
Elisa Di Giorgio, Marco Lunghi, Rosa Rugani, Lucia Regolin, Beatrice Dalla Barba, Giorgio Vallortigara and Francesca Simion
Developmental Science, 2019

Lateralized mechanisms for encoding of object. Behavioral evidence from an animal model: the domestic chick (Gallus gallus).
Rosa Rugani, Orsola Rosa Salva and Lucia Regolin
Frontiers in Psychology, 2014

Number-space mapping in the newborn chick resembles humans’ mental number line.
Rosa Rugani, Giorgio Vallortigara, Konstantinos Priftis and Lucia Regolin
Science, 2015

Rhesus monkeys (Macaca mulatta) map number onto space.
Caroline Beth Drucker and Elizabeth Merrit Brannon
Cognition, 2014

Rudimental numerical competence in 5-day-old domestic chicks (Gallus gallus): Identification of ordinal position.
Rosa Rugani, Lucia Regolin and Giorgio Vallortigara
Journal of Experimental Psychology: Animal Behavior Processes, 2007

Spatial representation of magnitude in gorillas and orangutans.
Regina ‘Reggie’ Gazes, Rachel Diamond, Jasmine Hope, Damien Caillaud, Tara Stoinski and Robert Hampton
Cognition, 2017

Spontaneous Spatial Mapping of Learned Sequence in Chimpanzees: Evidence for a SNARC-Like Effect
Ikuma Adachi
PLoS ONE, 2014

The mental representation of parity and number magnitude.
Stanislas Dehaene, Serge Bossini and Pascal Giraux
Journal of Experimental Psychology General, 1993

Visualised numerals.
Sir Francis Galton
Nature, 1880