Prenatal light exposure affects number sense and the mental number line in young domestic chicks.

Humans order numerosity along a left-to-right mental number line (MNL), traditionally considered culturally rooted. Yet, some species at birth show spatial-numerical associations (SNA), suggesting neural origins. Various accounts link SNA to brain lateralization but lack evidence. We investigated brain lateralization effects on numerical spatialization in 100 newborn domestic chicks. In ovo light exposure yielded strongly lateralized brains in half the chicks and weakly lateralized in the other half. Chicks learned to select the 4th item in a sagittal array. At the test, the array was rotated 90°, with left and right 4th items correct. Strongly lateralized chicks outperformed weakly lateralized ones when ordinal and spatial cues were reliable (experiment 1), but not with unreliable spatial cues (experiment 2). Moreover, only strongly lateralized chicks showed left-to-right directionality, suggesting the right hemisphere's key role in integrating spatial and numerical cues. We demonstrate that brain lateralization is fundamental for developing a left-to-right oriented SNA.