A new study has found that a chick’s ability to mentally organize numbers along a line from left to right is not learned but is instead a direct result of brain specialization that occurs before hatching. Researchers found that exposing chick eggs to light in the final days of incubation causes the two hemispheres of the brain to develop distinct functions, which in turn establishes an innate tendency for the chicks to count from left to right.
For many years, scientists and philosophers have debated the origins of the “mental number line,” a common intuition where people visualize smaller numbers on the left and larger numbers on the right. The prevailing theory suggested this was a cultural artifact, learned through years of reading and writing in a left-to-right direction.
However, this idea has been challenged by findings that pre-verbal infants and even some animals exhibit a similar spatial bias for numbers, suggesting a deeper, biological foundation. Researchers have hypothesized that this foundation lies in brain lateralization, the process where the left and right hemispheres of the brain become specialized for different cognitive tasks. While this connection seemed plausible, there was little direct experimental evidence to confirm that brain specialization actually causes this numerical mapping.