| This dissertation investigates the "younger is better" phenomenon in second language acquisition (SLA). The age-related difference in L2 ultimate attainment has long been an issue of heated debate in SLA research, owing largely to its unclear causal factors. In an attempt to identify causal factors of the age-related difference, the present study focuses on the roles of two potential factors---the Critical Period (CP) and first language (L1) influence---that have been suggested as the most potent universal constraints underlying the age-related difference.; Specifically, the study examined the L2 ultimate attainment of 112 Korean native speakers who immigrated to the United States at different ages. The participants' acquisition of two English linguistic features, passives and articles, was assessed via two different tasks, a grammaticality judgment task and a fill-in-the-missing-word task.; The study found that the CP and L1 influence play independent and concerted roles in determining the age-related difference. The results from both tasks confirmed the existence of the CP, supporting it as a factor differentiating the L2 ultimate attainment of younger and older starters. Further, the results from the grammaticality judgment task showed that the acquisition within the CP did not depend on L1-L2 similarity effects, whereas the acquisition outside the CP largely depended on L1-L2 similarity effects. This suggested that L1 influence, working in conjunction with the CP, impacts older starters' L2 ultimate attainment. However, the results from the fill-in-the-missing-word task did not support the role of L1 influence. Given the differential outcomes from the two tasks, the study partially attested to the interaction of the CP and L1 influence in generating the age-related difference.; The findings of the study add to the recent research view that no single factor can adequately account for the age-related difference. The study further highlights the importance of expounding the inter-relations of multiple factors involved in the age related difference and of specifying the role played by each factor in their interaction. |
