Cumulative L1–L2–L3 lexical similarity versus L2–L3 lexical similarity: What impacts learners’ L3 word knowledge and L3 word processing more?

We investigated how previous languages and learner individual differences impact L3 word knowledge. The participants were 93 L1-Polish learners of L2-English and L3-Italian. We tested participants’ knowledge of 120 L3-Italian words: 40 L2–L3 cognates, 40 L1–L2–L3 cognates, and 40 non-cognates, controlled for many item-related variables. The knowledge and online processing of the L3 words were measured by a test inspired by the Vocabulary Knowledge Scale and a lexical decision task (LDT), respectively. The results revealed that L1–L2–L3 cognates were known better than L2–L3 cognates, but L2–L3 cognates did not differ from non-cognates. Processing advantage was observed only for low-frequency triple cognates. Moreover, cognitive aptitudes predicted the speed of responding to the keywords in the LDT. However, they did not predict participants’ performance on the vocabulary test, where L3 proficiency effects prevailed. Our results suggest that L1–L2–L3 similarity is more conducive to learning than single-sourced L2–L3 similarity.