The aim of the present study is to revisit the old debate between rationalists and empiricists in relation to language development with new longitudinal data in hand. I show that when it comes to the development of a specific piece of linguistic knowledge, namely the distribution of the third person singular morpheme -s in child English, the generativist approach can satisfactorily account for the quirks observed in the longitudinal data presented herein. First, I argue that children are not conservative learners in the sense of Tomasello (2003), but they set parameters in the sense of Crain (1991). That is to say, child grammars may vary from the adult -significantly-, but the variation is conservatively limited by the hard-wired principles and parameters of Universal Grammar. I conclude that a parameter setting account of the development of functional categories is preferred as it attains explanatory adequacy with a minimal set of assumptions.

I then adopt Lasnik's (1995a) parametric account of verbal morphology, which distinguishes two types of Infl(ectional) items: affixal Infl and featural Infl. Furthermore, I use the same distinction to account for the development of sentential Neg(ation), as well, arguing that there are two parametric values associated with the Neg(ation) category: affixal Neg and featural Neg. In natural languages, the intersection of these values defines different grammars (e.g. Swedish vs. English, Middle English vs. Modern English). Based on the Principles and Parameter theory, I show that at any given point in development, innately hard-wired UG principles and parameters can accurately define child grammars the same way they define any natural language. I argue that longitudinal evidence suggests that these parametric values are hard wired as those options are explored by four different English-speaking 2-year-old children. Thus, I conclude that language development is better understood as language change driven by parameter setting and re-setting.