Second language learners' performance in mathematics: Disentangling the effects of academic language features

•We examined differential item functioning (DIF) of math items against second language learners.•Academic language increases the difficulty of math items for second language learners.•Descriptive, lexical, and grammatical features have unique effects on DIF.•There are no effects of specialised mathematical vocabulary on DIF.•Intercorrelations among academic language features are substantial.

Several studies have shown that the linguistic complexity of items in achievement tests may cause performance disadvantages for second language learners. However, the relative contributions of specific features of linguistic complexity to this disadvantage are largely unclear. Based on the theoretical concept of academic language, we used data from a state-wide test in mathematics for third graders in Berlin, Germany, to determine the interrelationships among several academic language features of test items and their relative effects on differential item functioning (DIF) against second language learners. Academic language features were significantly correlated with each other and with DIF. While we found text length, general academic vocabulary, and number of noun phrases to be unique predictors of DIF, substantial proportions of the variance in DIF were explained by confounded combinations of several academic language features. Specialised mathematical vocabulary was neither related to DIF nor to the other academic language features.