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We investigate the origin of differences in the word frequency effect between native speakers and second-language speakers. In a large-scale analysis of English word identification times we find that group-level differences are fully accounted for by the individual language proficiency scores. Furthermore, exactly the same quantitative relation between word frequency and proficiency is found for monolinguals and three different bilingual populations (Dutch-English, French-English, and German-English). We conclude that the larger frequency effects for second-language processing than for native-language processing can be explained by within-language characteristics and thus need not be the consequence of "being bilingual" (i.e., a qualitative difference). More specifically, we argue that language proficiency increases lexical entrenchment, which leads to a reduced frequency effect, irrespective of bilingualism, language dominance, and language similarity.
Diependaele et al. (Tue,) studied this question.