For example, a recent study co-authored by one of us (Ibbotson) showed that children’s ability to produce a correct irregular past tense verb — such as “Every day I fly, yesterday I flew” (not “flyed”) — was associated with their ability to inhibit a tempting response that was unrelated to grammar. (For example, to say the word “moon” while looking at a picture of the sun.) Rather than memory, mental analogies, attention and reasoning about social situations getting in the way of children expressing the pure grammar of Chomskyan linguistics, those mental faculties may explain why language develops as it does.

As with the retreat from the cross-linguistic data and the tool-kit argument, the idea of performance masking competence is also pretty much unfalsifiable. Retreats to this type of claim are common in declining scientific paradigms that lack a strong empirical base — consider, for instance, Freudian psychology and Marxist interpretations of history.

Even beyond these empirical challenges to universal grammar, psycholinguists who work with children have difficulty conceiving theoretically of a process in which children start with the same algebraic grammatical rules for all languages and then proceed to figure out how a particular language — whether English or Swahili — connects with that rule scheme.

Linguists call this conundrum the linking problem, and a rare systematic attempt to solve it in the context of universal grammar was made by Harvard University psychologist Steven Pinker for sentence subjects. Pinker’s account, however, turned out not to agree with data from child development studies or to be applicable to grammatical categories other than subjects. And so the linking problem — which should be the central problem in applying universal grammar to language learning — has never been solved or even seriously confronted.