This study assesses the impact of internal and external factors on very young EFL learners in an instructional setting. 71 child English learners in China (onset age: 2;0 - 5;6) were involved: their receptive vocabulary, productive vocabulary and receptive grammar were taken as outcome variables, and age of onset, short-term memory, nonverbal intelligence, English input quantity and quality, English use, and maternal English level were taken as predictive variables. Multiple regression analyses, verified by Bayes factor comparisons, revealed that the total amount of school input and home English media environment were significant predictors for all of three aspects of English proficiency, with each aspect having different additional significant predictors. Both internal factors (e.g., age of onset) and external factors (e.g., English input quantity) played an important role, but in contrast to similar studies (e.g., Paradis, 2011) focusing on a L2 naturalistic setting, external factors explained more variance of English proficiency measures.
