, 2009). A similar observation was made with a group of adult homesigners from Nicaragua (Spaepen et al., 2011): In a series of set-reproduction tasks, homesigners used one-to-one correspondence strategies only rarely, and when Dorsomorphin research buy they did so, they used it by mapping their fingers (the constituents of their number signs) to objects, never by mapping

two sets directly onto each other, a seemingly simpler strategy. Understanding how words (or, in the case of homesigners, fingers) stand in one-to-one correspondence with objects while counting may be the first step that leads to a more general understanding of one-to-one correspondence relations, and in particular of how one-to-one correspondence warrants exact numerical equality. Our findings shed light both on the extent and the limits of children’s numerical knowledge, before they master the meanings of all the number words they use in counting. Children who have not mastered the exact numerical meanings of “five” and “six” are able to use one-to-one correspondence cues to reconstruct a set of exactly five or six objects, even when the sets are moved around, rearranged in space, and kept out of view for some time, and even if one individual is first subtracted and then added back to the set, as long as the identity INCB024360 manufacturer of the items

forming the sets is not modified. However, children do not know how set transformations that change the individual members affect the way sets can be measured by one-to-one correspondence. Hence, before children acquire symbols for exact number, one-to-one correspondence defines a relation of identity between sets: a relation that is not limited to approximate numerical equality but falls short of exact numerical Thymidylate synthase equality. Furthermore, children do not understand how one-to-one mappings interact with the addition of one, i.e. the successor function. At 3 years of age, the child’s state of knowledge for number thus corresponds to the initial stage of Russell–Frege’s formal definition of cardinal integers:

they have a relation of set identity, but yet have not figured out how this notion interacts with basic operations, and how the numbers can be ordered in a list structured by a successor function. This research was supported by grants from NIH (HD 23103) and NSF (0633955) to E.S.S., and by a postdoctoral grant from the Fyssen Foundation and a Starting Grant from the European Research Council (MathConstruction 263179) to V.I. We thank Ariel Grace, Kate Ellison and Konika Banerjee for help in data collection; Amy Heberle and LeeAnn Saw for help in offline data recoding; Renée Baillargeon, David Barner, Susan Carey, Lola de Hevia, Lisa Feigenson, Justin Halberda, Mathieu Le Corre, Peggy Li, T.R. Virgil, and one anonymous reviewer for helpful discussions and comments throughout the course of the project; and all the parents and children who kindly participated in the research.