Before learning the cardinal principle (knowing that the last word reached when counting a set represents
the size of the whole set), children do not use number words accurately to label most set sizes.
However, it remains unclear whether this difficulty reflects a general inability to conceptualize and communicate
about number, or a specific problem with number words. We hypothesized that children’s gestures
might reflect knowledge of number concepts that they cannot yet express in speech, particularly for
numbers they do not use accurately in speech (numbers above their knower-level). Number gestures are
iconic in the sense that they are item-based (i.e., each finger maps onto one item in a set) and therefore
may be easier to map onto sets of objects than number words, whose forms do not map transparently
onto the number of items in a set and, in this sense, are arbitrary. In addition, learners in transition with
respect to a concept often produce gestures that convey different information than the accompanying
speech. We examined the number words and gestures 3- to 5-year-olds used to label small set sizes
exactly (1–4) and larger set sizes approximately (5–10). Children who had not yet learned the cardinal
principle were more than twice as accurate when labeling sets of 2 and 3 items with gestures than with
words, particularly if the values were above their knower-level. They were also better at approximating
set sizes 5–10 with gestures than with words. Further, gesture was more accurate when it differed from
the accompanying speech (i.e., a gesture–speech mismatch). These results show that children convey
numerical information in gesture that they cannot yet convey in speech, and raise the possibility that
number gestures play a functional role in children’s development of number concepts.