eLife Sep 10 2025 Lateralization of the brain – the tendency for the left and right hemispheres to specialize in different functions – underlies the development of a left-to-right mental number line, according to a study in newborn chicks.  The study, published as a Reviewed Preprint in eLife and appearing today as the final version, is described by the editors as fundamental. They say the evidence presented is compelling and the results will be of interest to researchers studying numerical