Not All Numbers Were Created Equal: Evidence the Number One is Unique

Children universally acquire the meaning of the words one, two, and three in that order. While research has focused on how children acquire this knowledge and what this knowledge represents, the question of why children learn small number words in order has been comparatively neglected. One possibility is that children’s working memory capacity to represent cardinality is limited; they can represent one item but not more than that, at least initially in development. Another possibility is that children can represent cardinality of sets up to three items, but other factors (e.g., linguistic, environmental, or algorithmic) promote their learning of the word one prior to the words two and three. In this study, we distinguish these two competing hypotheses using a non-verbal anticipatory looking task in 14- to 23-month-olds (N=46), where the infants were assessed on their ability to form implicit category structures for sets of one, two, and three items. Infants (regardless of age) were able form a category for sets with one item but not for sets with two or three items, as evidenced by their anticipatory looking behavior. The results are consistent with the hypothesis that limited working memory capacities to represent cardinalities of sets greater than one explains the universal pattern of children’s acquisition of small number words. Implications of our results for parallel individuation, number acquisition theories, and the development of working memory resources are discussed.