Linguistic distance effects on country TOEFL scores between 2005 and 2020 and the effect of morphological distance in particular

We analysed aggregated TOEFL scores from over 15 million non-immersion learners across 127 countries and 65 first languages (L1s), spanning a 16-year period (2005–20). This extensive dataset enabled us to investigate not only the effects of various country-level characteristics – including linguistic distances – but also the dynamic nature of these effects over time. To capture complex, non-linear interactions, we applied a generalized additive mixed regression model (GAMM), with particular focus on three linguistic distance measures: morphological, lexical, and phonological. This approach allowed us to map the evolving role of morphological distance in relation to the other two dimensions. All three linguistic distance measures significantly contributed to explaining variation in L2 English proficiency at the country level. Greater linguistic distance was consistently associated with lower TOEFL scores. Morphological distance provided a distinct and competitive contribution alongside lexical and phonological distances. Importantly, all three measures interacted significantly with the year of testing, indicating that the impact of linguistic distance is not static but evolves over time. These temporal changes likely reflect the increasing global presence and dominance of English as a second language. Specifically, morphological and phonological properties gained importance at higher levels of L2 proficiency, while the influence of lexical distance gradually diminished.