This review article selects and elaborates on the important issues of adult second language acquisition research in the second decade of the twenty-first century. The fundamental question of whether adult second language acquisition and child first language acquisition are similar or different is addressed throughout the article. The issues of a critical period for acquisition, the importance of the linguistic input, and processing are discussed. Generative as well as usagebased perspectives are considered. Future research concerns and promising areas of investigation are proposed.Key words: Critical Period Hypothesis, the Bilingual Turn, linguistic input, generative approaches to SLA, usage-based approaches to SLA, processing 1 I am grateful to three anonymous reviewers for giving me excellent suggestions on clarity as well as substance, Iregret that I could not incorporate even more of their suggestions than I did. Thanks also go to Tiffany Judy, who read the whole text in a preliminary version and helped me to enhance the accessibility of the exposition. All errors and opinions are mine.
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IntroductionIn every review of an extensive body of research (such as, for example, the body of work investigating adult second language acquisition, SLA, or L2A), the author is inevitably faced with the hard choice of what issues to cover, from what perspective, and whether to go for a chronological or for a current-situation approach. In making these choices, then, I have been guided primarily by two considerations. The first is that the research reviewed should fall into the remit of the journal Linguistic Approaches to Bilingualism; that is, it should represent formal and cognitive approaches to SLA that are based in a grammatical or psychological theory of language. This means that I will not attempt to cover much of the interesting research on aptitude, individual differences, motivation and the social factors that affect SLA, that is, the acquisition process from a learner's perspective. I will focus instead on what system of mental representations (mental grammar) learners build in the acquisition process, what prompts learners to go from one (relatively) steady state to another; and how they use this mental grammar in their language production and comprehension.Secondly, and not only for space reasons, I have mostly chosen to skip chronology and to present issues of current concern. Thus, for example, I am not going to spend much time on the issue of what constitutes the initial state of SLA, since it seems that not too many researchers have been inspired to make new claims about this matter in the last decade. This current lack of research interest in the topic does not mean that we have solved the matter to everyone's satisfaction, nor that we have a definitive answer to the question "What constitutes the initial state of L2A?" (Meisel, 2011:91). Perhaps it is altogether too optimistic to expect that answers to seminal questions within a scientific discipline will inevitably reach some sort of relative co...