Summary Over the past decade there has been a surge of neuroscientific interest regarding the mechanisms that underpin human pro-social decision-making. However, the exact role played by numerical cognition in social decision-making remains unclear. This is surprising given the explicitly numerical nature of the dictator game task, which has been almost ubiquitously employed to examine the neural basis of pro-social behavior. We hypothesized that numerical magnitude allocation may play a critical role in the choices formulated during the dictator game task, and found that innate numerical biases and pro-social choices strongly correlated. Numerical magnitude biases towards lower numbers were associated with less favourable pro-social choices, whereas biases towards higher numbers were associated with greater pro-social choices. We proceeded to subliminally induce bi-directional modulation of numerical magnitude perception using a combined visuo-vestibular stimulation paradigm, and subsequently assessed dictator game performance. Strikingly, we found that altering numerical magnitude allocation led to corresponding changes upon an individual's pro-social choices during the dictator game task. Taken together, these findings strongly suggest that tasks with an explicit numerical component, such as the dictator game, are directly dependent upon numerical biases. Our findings have significant implications for our understanding of performance on the standard dictator game paradigm and future studies investigating pro-social decision-making.