Does productive agreement morphology increase sensitivity to agreement in a second language?

Adult language learners have variable performance with subject-verb number agreement. But it is unclear whether their performance additionally depends on the availability of agreement morphology in their first language. To address this question, we conducted a self-paced reading task comparing different speaker groups: (a) first vs. second language speakers of German; (b) intermediate-to-advanced German learners whose first language had more or less productive number agreement morphology (Spanish vs. English). Two manipulations were used to diagnose number processing: agreement violations and agreement attraction. Our results showed decreased sensitivity to agreement violations in language learners, irrespective of the morphological productivity of their first language. Meanwhile, differences in attraction effects were inconclusive in all between-group comparisons. We suggest that second language variability with subject-verb agreement is unlikely to result from increased retrieval interference – the effect underlying attraction. Instead, variable performance more likely arises because learners have difficulties in the real-time mapping of inflectional morphemes to syntactic features.