Children's Cardinality Knowledge Isn't Always Beneficial

A milestone in cognitive development is understanding numerals to represent the exact number of discrete items in a set (i.e., the cardinal principle). This development has received much attention, but little is known about its relation to understanding numbers as measures of continuous quantity (e.g., “six blocks long” versus “six blocks”). To investigate this, 90 children were asked to complete two tasks: a give-a-number task, to assess cardinality knowledge, and a novel give-a-line task, to assess measurement knowledge. As expected, accuracy was greater on the give-a-number task than the give-a-line task. More unexpectedly, the quality of performance on the give-a-number task was as often negatively associated with quality of performance on the give-a-line task as it was positive correlated. Specifically, when asked to create a line N-blocks long, children who gave only approximately correct answers on the give-a-number task often outperformed children who gave exactly correct answers on the same task. These findings indicate that an approximate-and purportedly less mature-understanding of number possesses the hidden strength of being more flexible and suitable for measuring length.