In this article, we propose that the Root Infinitive (RI) phenomenon in child language is best viewed and explained as the interaction between morphological learning and syntactic development. We make the following specific suggestions: The optionality in RI reflects the presence of a grammar such as Chinese which does not manifest tense marking. The gradual elimination of the nontense-marking grammar is facilitated by the learning of the morphosyntactic system of the target language. Quantitative differences in the input data among morphosyntactic systems result in the cross-linguistic variation in the RI phenomenon. More broadly, we aim to demonstrate that quantitative aspects of language learning data and concrete mechanisms of the language learning process can play an important role in the generative approach to language acquisition. In Section 2, we give a brief overview of the RI literature along with some methodological remarks regarding the explanation of the phenomenon. In Section 3, we lay out our theory of morphosyntactic learning and the broader variational approach to language acquisition. Our empirical work focuses on the development of tense in Spanish, French, and English. In Section 4 we show through corpus study of child-directed speech that differences in the morphosyntactic systems of these three languages explain the brief RI stage in Spanish acquisition, the prolonged RI stage in English acquisition, as well the intermediate status of the RI stage in French. In Section 5, we discuss how our approach relates to various findings established in the previous literature on RI. Section 6 concludes with a general discussion of the proper role of the input data in theories of language acquisition.