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Summary
Regardless of cultural background and mathematical training, all humans have an intuitive sense of numerical magnitude (numerosity). We share with various nonhuman animals the ability to discriminate among different sets of quantities (1), but one aspect of number processing is commonly assumed to be uniquely human (2): the consistent mapping of increasing quantities along the horizontal extension of space—that is, the construction of a mental number line. On page 534 of this issue, Rugani et al. (3) show that 3-day-old domestic chicks (see the photo) associate small numerosities with the left side, and large ones with the right side, of a given space (see the figure, panel A). The results show that newborn chicks can understand both relative and absolute quantities. They also suggest that the brain may be prewired in how it relates numbers to space.
