No two children, even those learning what is regarded as the same language, have exactly the same experiences of language. Most children have enbtirely different experiences. And yet they all converge on the same grammar, a shared competence, and agree or disagree about plays on words and other sorts of jokes and the exact meaning of business contracts and political resolutions and agreements. And if they don’t agree, they can, and often do, argue about the meanings. It is like a meeting of minds.

This is shown most dramatically in the language development of two normally-developing brothers, Joe and Arthur, two and half years apart in age, analysed more fully and in mire detail in the proposal here,. At around two years and ten months,  Joe said “I want to stand on the chair to see what’s happening”. At the same age,  Frank says “I want to sit where Joe’s been sitting.” In both children, these were thes first sentences with multiple embeddings, in both cases with the most deeply embedded clause introduced by an Wh word, what in Joe’s case, and where in Frank’s. In both cases, the structure is completely grammatical with a contracted auxiliary ‘s forming a tensed structure.

Obviously, what Joe was saying was an important part of Frank’s language experience. But while Frank may have taken on board many of Joe’s interests, it does not seem plausible that this extended to such trivial curiosities.

The exactness of the similarity between these structures (only noticed forty years later when the data came to be ansalysed), and their complete grammaticality at a point when the first embeddings were just first appearing, does strongly suggest that there is some significance in them. By the proposal here, these structures signal a critical point in the mastery of ‘Universal Grammar’.

By the same token, this is only possible if the essential structures, are well defined, and not some random chunks of language experience, as proposed by Mike Tomasello (2003), Ben Ambridge (2004), and others.