Training "Zero" in preschoolers: Fast referential learning, slow relational integration

Children's understanding of the number word "zero" typically lags behind positive numbers. According to stage theories, they first interpret "zero" as "nothing" and only later acquire its cardinal meaning and relation to other numbers. This protracted sequence may reflect either limited exposure or the unique conceptual difficulties posed by "zero." The present study examined (1) if a brief training that matched an empty plate repeatedly with the number word "zero" was enough for children (referred to as "zero-non-knowers") to acquire both the "nothing" and the cardinal meaning of "zero"; (2) whether a second training session associating "zero" to a null quantity of a specific kind (e.g., a plate that contains "three apples" but "zero bananas") supported the learning of its cardinal meaning; and (3) whether learning either of these two meanings supports the ability to make numerical comparisons (e.g., that "two" is "more" than "zero"). We found that children aged 3 to 5 years (M = 3.89, SD = 0.58) required fewer than 10 trials to map "zero" to an empty plate in the first training session, but that they required additional trials to associate "zero" to a null quantity of a specific kind in the second training phase, suggesting that many children in the first phase associated "zero" with nothing, rather than with zero items of a specific kind. However, although children quickly learned the referential meanings of "zero," this training had no impact on their ability to compare it to other numbers, suggesting that this ability develops separately, compatible with stage theory accounts. Thus, although limited exposure to "zero" may be one important factor explaining its delayed acquisition, integrating it into the ordered set of number words may involve additional conceptual change.