The overarching goal of the dissertation is to illustrate the relevance of learner corpus research to the field of second language acquisition (SLA). The possibility that learner corpora can be useful in mainstream SLA research has a significant implication given that they have not been systematically explored in relation to SLA theories. The thesis contributes to building a methodological framework to utilize learner corpora beneficially to SLA and argues that learner corpus research contributes to other disciplines. This is achieved by a series of case studies that quantitatively analyze individual variation and the role of native language (L1) in second language (L2) development of English grammatical morphemes and explain the findings with existing SLA theories.

The dissertation investigates the L2 development of morphemes based on two largescale learner corpora. It first reviews the literature and points out that the L2 acquisition order of English grammatical morphemes that has been believed universal in SLA research may, in fact, vary across the learners with different L1 backgrounds and that individual differences in morpheme studies have been relatively neglected in previous literature. The present research, thus, provides empirical evidence testing the universality of the order and the extent of individual differences.

In the first study, the thesis investigates L1 influence on the L2 acquisition order of six English grammatical morphemes across seven L1 groups and five proficiency levels. Data drawn from approximately 12,000 essays from the Cambridge Learner Corpus establish clear L1 influence on this issue. The study also reveals that learners without the equivalent morpheme in L1 tend to achieve an accuracy level of below 90% with respect to the morpheme even at the highest proficiency level, and that morphemes requiring learners to learn to pay attention to the relevant distinctions in their acquisition show a stronger effect of L1 than those which only require new form-meaning mappings. The findings are interpreted under the framework of thinking-for-speaking proposed by Dan Slobin.

Following the first study, the dissertation exploits EF-Cambridge Open Language Database (EFCamDat) and analyzes the developmental patterns of morphemes, L1 influence on the patterns, and the extent to which individual variation is observed in the development. Based on approximately 140,000 essays written by 46,700 learners of 10 L1 groups across a wide range of proficiency levels, the study found that (i) certain developmental patterns of accuracy are observed irrespective of target morphemes, (ii) inverted U-shaped development is rare irrespective of morphemes, (iii) proficiency influences the within-learner developmental patterns of morphemes, (iv) the developmental patterns at least slightly vary depending on morphemes, and (v) significant individual variation is observed in absolute accuracy, the accuracy difference between morphemes, and the rate of development. The findings are interpreted with dynamic systems theory (DST), a theory of development that has recently been applied to SLA research. The thesis further examines whether any systematic relationship is observed between the developmental patterns of morphemes. Although DST expects that their development is interlinked, the study did not find any strong relationships between the developmental patterns. However, it revealed a weak supportive relationship in the developmental pattern between articles and plural -s. That is, within individual learners, when the accuracy of articles increases, the accuracy of plural -s tends to increase as well, and vice versa.