Lead
"She's three and still can't count" is usually not a cause for concern — and the reason is more interesting than it might sound. Hearing "one, two, three" come out of a child's mouth does not necessarily mean that child is counting, and conversely, a child who never says those words aloud may be quietly building a genuine understanding of quantity.
Number concept development runs considerably deeper than the surface skill of reciting a sequence. It begins in infancy with an intuitive grasp of amount, works its way through the gradual acquisition of counting principles across ages three and four, and arrives — in a moment researchers have spent decades trying to characterize — at the understanding that the last number you say when counting a group represents the whole group. This article lays out what the evidence currently shows about how that journey unfolds.