<p>Adults tend to systematically associate numerical information with spatial positions, such as associating smaller numbers with the left and larger numbers with the right side of space. These spatial–numerical associations were initially attributed to cultural factors such as reading direction, but evidence from non-human animals suggests a biological foundation. In this Review, we examine behavioural evidence to clarify the origins of spatial–numerical associations and how cultural and biological factors shape their existence and direction. First, we discuss how specific tasks and stimulus types constrain the implications of spatial–numerical association studies. Then we review findings from non-human animals and human infants that suggest early biological predispositions for a left-to-right mapping of numerical information onto space. Next, we consider findings from adults with different written language experience, which highlight the role of cultural experience. Finally, we speculate on possible mechanisms underlying an implicit form of non-symbolic spatial–numerical associations and propose directions for future research.</p>