ABSTRACT
The central problem to be addressed in this chapter is whether the grammars of interlanguages (IL) adhere to the constraints of UG, or whether in some cases they fall outside of these constraints. A related problem to be addressed involves the type of data and argumentation that bear on this problem. Much of the literature on the central problem, whether ILs obey UG, claims that L2 grammars do conform to the principles of UG, though there has been room for disagreement (e.g., Schachter, 1988; Hagen, this volume, chapter 4). The vast majority of studies addressing the related question, that of the type of data needed to support the claim, have reported aggregate data rather than data from individuals. In this chapter, it is argued that data from individuals, rather than group data, are necessary to test whether IL grammars adhere to the principles of UG.
