The mental number line has a biological basis. A commentary on: Number-space mapping in the newborn chick resembles humans' mental number line by Rugani, R., Vallortigara, G., Priftis, K., & Regolin, L. (2015). Science, 347(6221), 534-536. - Caroline Beth Drucker; Elizabeth M Brannon (Frontiers in Psychology, 2015)