Experiential Origins of the Mental Number Line

People map numbers onto horizontal space, forming an implicit mental number line (MNL). The direction of the MNL, which varies across cultures, has often been attributed to the direction of reading and writing words. Yet, this proposal is neither clearly motivated nor well supported by experimental data. Here we tested the hypothesis that fingercounting habits can determine the direction of the MNL. Americans were trained to count on their fingers from left to right or from right to left. After rightward counting, participants showed implicit associations of small numbers with left space and large numbers with right space, typical for Americans. After leftward counting, this space-number association was extinguished, overall, and was qualitatively reversed in a significant proportion of the individual participants. A few minutes of finger counting experience can redirect the MNL, supporting a causal role for finger counting in the acquisition and maintenance of culture-specific mental number lines.