Infants use different mechanisms to make small and large number ordinal judgments.

Previous research has shown indirectly that infants may use two different mechanisms--an object tracking system and an analog magnitude mechanism--to represent small (<4) and large (≥4) numbers of objects, respectively. The current study directly tested this hypothesis in an ordinal choice task by presenting 10- to 12-month-olds with a choice between different numbers of hidden food items. Infants reliably chose the larger amount when choosing between two exclusively small (1 vs. 2) or large (4 vs. 8) sets, but they performed at chance when one set was small and the other was large (2 vs. 4) even when the ratio between the sets was very favorable (2 vs. 8). The current findings support the two-mechanism hypothesis and, furthermore, suggest that the representations from the object tracking system and the analog magnitude mechanism are incommensurable.