Educational policies that impact second language (L2) learners—a rapidly-growing group—are often enacted without consulting relevant research. This review synthesized research regarding optimal conditions for L2 acquisition, facilitative L2 learner and teacher characteristics, and speed of L2 acquisition, from four bodies of work—foreign language education, child language research, sociocultural studies, and psycholinguistics—often overlooked by educators. Seventy-one peer-reviewed journal articles studying PK-12 L2 learners met inclusion criteria. Findings included: 1) Optimal conditions for L2 learners immersed in a majority-L2 society include strong home literacy practices, opportunities to use the L2 informally, well-implemented specially-designed L2 educational programs, and sufficient time devoted to L2 literacy instruction, whereas L2 learners with little L2 exposure require explicit instruction to master grammar; 2) L2 learners with strong L2 aptitude, motivation, and first language (L1) skills are more successful; 3) Effective L2 teachers demonstrate sufficient L2 proficiency, strong instructional skills, and proficiency in their students’ L1; 4) L2 learners require 3-7 years to reach L2 proficiency, with younger learners typically taking longer but more likely to achieve close-to-native results. These findings, even those most relevant to education, are not reflected in current US policy. Additional research is needed on the characteristics of successful or unsuccessful L2 learners and L2 teachers. Such research should attend systematically to the differences between L2 learning in maximal versus minimal input settings; whereas the psycholinguistic challenges of L2 learning might be common across settings, the sociocultural and interactional challenges and opportunities differ in ways that can massively impact outcomes.
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