A universal left-to-right bias in number-space mapping across ages and cultures

Number and space are inherently related. For decades, authors have collected evidence showing that numbers are aligned to a so-called “mental number line”, which is malleable and affected by cultural factors. However, preverbal human and non-human animals also map numerosities into space, in a consistent left to right direction. These contrasting pieces of evidence raise the question of whether Space Number Associations (SNA) are culturally or biologically determined. Here, we investigated Italian adults, Italian preschoolers and Himba adults to determine whether cultural influences are necessary for SNA to emerge. We found that, when explicitly asked to order numerosities, only Italian adults showed a consistent left to right preference, while preschoolers and Himba adults did not have a consistent preference for one direction or the other. On the other hand, in a numerosity comparison task, all groups performed better when small numerosities were presented in the left hemispace. These results suggest that SNA is not a unique phenomenon, but rather is dissociable in two components: a universal one, biologically predisposed and left-to-right oriented, and an acquired one, culturally dependent and not fixed in orientation.