What works better for the learning of concrete and abstract words: teachers' L1 use or L2-only explanations?

This study examined the differential effects of teachers' first language (L1) use and second language (L2)-only explanations on Chinese-speaking adults' acquisition of concrete and abstract English words. A quasi-experimental research design was used, in which 50 participants were assigned to an L1-use condition, 50 participants were assigned to an L2-only use condition, and another 48 participants served as a comparison group. Assessed with a pre-test, immediate post-test, and delayed post-test, each group's knowledge of the target words was compared with that of the other groups over time. The results suggest that teachers' L1 use, compared to L2-only explanations, might lead to greater vocabulary gains in immediate and delayed recall. Possible explanations for these findings are proposed.