Núñez, Rafael, and Fias, Wim. (2015). "Ancestral Mental Number Lines: What Is the Evidence?." Cognitive Science.
Over the last two decades substantial efforts have been made to investigate the question of whether the building blocks of human mathematical concepts ultimately have their origins in biological evolution. A relevant case study is the “mental number line” hypothesis, which states that numbers are represented in the brain as spatial entities along a mental line, yielding behavioral manifestations. Some developmental (de Hevia & Spelke, 2009, 2010), cross-cultural (Dehaene, Izard, Spelke, & Pica, 2008a), and comparative (Drucker & Brannon, 2014) studies have suggested that number-to-space mappings—underlying mental number lines—are biologically endowed universals, emerging independently of language and culture. Recently, going further, Rugani, Vallortigara, Priftis, and Regolin (2015) have argued that newborn domestic chicks (Gallus gallus) map numbers to space resembling humans’ mental number line, and they claimed that “spatial mapping of numbers from left to right may be a universal cognitive strategy available soon after birth” (p. 536). After training newborn chicks to circumnavigate a centered panel depicting a target numerosity (5 elements for some chicks, 20 for others), the researchers allowed the chicks to explore an environment containing two panels—to the left and to the right, displaying identical numerosities either smaller or greater than the target (2 or 8 elements, and 8 or 32, respectively). The authors reported that around 70% of the time the chicks preferred the left panel when the numerosity was smaller than the target and the right one when it was greater. They interpreted these results as evidence that there is a left-to-right number-to-space mapping in newborn chicks that resembles humans’ mental number line. But do the data really support these claims?