Although children can combine words to express meanings like ‘red house’ or ‘two cats’, they do not represent number words compositionally (e.g. ‘twenty-six’ as 6+20) until late in development. One reason for this might be that smaller number words in most languages are not composed via rules, limiting children’s exposure to the syntactic structure underlying larger number words. However, historically, many languages have featured number systems with smaller bases or anchors, thus relying more on rules to represent smaller numbers. Might children acquire rules for combining numbers sooner, when exposed to evidence of such rules? We explored this in two experiments. In experiment 1, children learned a system anchored at 2, and many children could compose small number words using a conjunctive rule—e.g. ‘two-and-one apples’. Some older children could also comprehend novel multiplicative expressions—e.g. ‘two twos of bananas’. Experiment 2 found that children applied additive rules even to a system of novel number words (‘Monkey numbers’) anchored at three. We suggest that children can acquire rules for composing number words when made salient in their input, sometimes even before they learn how to accurately count large sets. This article is part of the theme issue ‘A solid base for scaling up: the structure of numeration systems’.
Holt et al. (2025) studied this question.