Theoretical and empirical issues in the study of implicit and explicit second-language learning introduction

There are good theoretical and educational reasons to place matters of implicit and explicit learning high on the agenda for SLA research. As for theoretical motivations, perhaps the most central issue in SLA theory construction in need of explanation is the differential success in one's first language (L1) and in one's second language (L2). Although acquisition of an L1 results in full mastery of the language (provided that children are exposed to sufficient quantities of input and do not suffer from mental disabilities), learners of an L2—even after many years of L2 exposure—differ widely in level of attainment. How can we explain universal success in the case of L1 acquisition and differential success in the case of L2 acquisition? Among the many explanations that have been proposed, including brain maturation and brain adaptation processes (critical period), access to Universal Grammar, L1 interference, and sociopsychological factors (see Hyltenstam & Abrahamsson, 2003, for a review), one finds explanations that involve the notions of implicit and explicit learning. Scholars working in different disciplines, in different theoretical schools, and sometimes using different terminology have argued that L1 acquisition (or at least the acquisition of L1 grammar) relies principally on processes of what we might now call implicit learning, whereas the acquisition of an L2 often relies on both implicit and explicit learning (Bley-Vroman, 1991; DeKeyser, 2003; N. Ellis, this issue; R. Ellis, 2004; Krashen, 1981; Reber & Allen, 2000).