Familiarity and literal plausibility, not transparency, drive idiom comprehension in adult readers with low literacy skills in Easy German

Introduction Easy German (Leichte Sprache) is a rule-guided variety of German intended to increase the accessibility of written information for people with cognitive disabilities and adults with low literacy skills. Although existing guidelines often discourage the use of idioms, experimental evidence on idiom processing in these target groups remains scarce. We investigated how literacy, functional familiarity, transparency, and literal plausibility jointly shape idiom comprehension and preferences in adult low-literacy readers. Methods In a norming study with literate German speakers (Experiment 1; Prolific; 71 verb-phrase idioms), we obtained seven-point ratings of transparency and literal plausibility (reverse-coded so that higher values indicate a more plausible literal scene) and selected 24 idioms to instantiate a 2 × 2 design (high/low transparency × high/low plausibility). Literacy in the target group was assessed with otu.lea (Experiment 2) and transformed into a quasi-continuous index (0–100). In Experiment 3 (no context), 30 low-literacy adults explained each idiom, providing a measure of functional familiarity. In Experiment 4 (same cohort after ~6 months; n = 26), idioms were embedded in Easy German contexts and tested using an endorsement task (idiom vs. paraphrase vs. distractor) and a ranking task (gold/silver/bronze). Results Mixed-effects models showed that, in Experiment 3, figurative accuracy increased with literacy (β = 0.028, p = 0.0019) and was related to literal plausibility (β = 0.569, p < 0.001), with a strong transparency × plausibility interaction (β = −1.457, p < 0.001): idioms that were both transparent and literally plausible yielded the lowest figurative accuracy. In Experiment 4, idiom endorsement was predicted by functional familiarity from Experiment 3 (β = 0.702, p = 0.025), while ranking responses showed robust distractor rejection and no reliable disadvantage for idioms relative to meaning-consistent paraphrases. Discussion Overall, idiom accessibility in low-literacy adults appears to be driven primarily by literacy, functional familiarity, and literal-figurative competition indexed by plausibility, whereas transparency does not emerge as a reliable stand-alone facilitator. These findings challenge Easy German recommendations that prioritize transparency and instead support applied strategies centered on audience familiarity, low literal plausibility, and contextual or paraphrastic scaffolding.