Stephen Crain (C) & Rosalind Thornton (T) have garnered a well-deserved reputation for their unwavering commitment to language learnability as a constraint not only on theories of child language and language development but also on experimental design and the interpretation of experimental findings. In his well-known defense of children's early knowledge of syntactic constraints, Crain (1991) argued for the widely-held position that the best solution to the learnability problem is to assume that grammatical knowledge which cannot be learned on the basis of experience is specified in advance as part of the human biological endowment for language in the form of a UNIVERSAL GRAMMAR (UG) (Chomsky, 1965). With respect to experimental design, C&T have strongly maintained that even young children know UG constraints but perform poorly in some experiments-due to the extralinguistic demands associated with experimental tasks, particularly those involved in presupposition accommodation and complex response planning. C&T specifically design their experiments to reduce the impact of extralinguistic demands on children's linguistic performance while at the same time providing felicitous environments for adultlike performance.
Journal of Child Language, 2004, Vol 31, Issue 2, p. 431-457