Singular–plural asymmetry in L2 English number processing: A sentence-picture matching study of Japanese learners of English

Aims and Objectives: This study investigates whether Japanese learners of English automatically map English number morphology onto conceptual numbers in both directions. Two research questions guide the study: RQ1. Mismatch costs: Do L2 learners show reaction-time slowdowns when: (a) A singular noun is paired with a plural picture? (b) A plural noun is paired with a singular picture? L1 speakers serve as a baseline to establish expected mismatch effects. RQ2. Directional asymmetry: If mismatch costs occur, are they equivalent in both directions, or is there a systematic asymmetry? Methodology: The study employed a sentence-picture matching task that crossed noun number (singular vs. plural) with picture condition (match vs. mismatch), extending previous unidirectional designs to test both directions of number mapping in L2 processing. Reaction times (RTs) served as the primary dependent measure. Data and Analysis: Thirty-two L1 English speakers and 96 Japanese L2 learners (CEFR B1–B2) each completed 200 trials (80 target items and 120 fillers). Incorrect trials were excluded, and extreme RTs were trimmed before the analysis. RTs were analyzed using inverse Gaussian generalized linear mixed-effects models, with sentence length and trial order included as covariates. Findings: L1 speakers showed significant RT slowdowns for both mismatch types, confirming automatic singular–plural mapping. In contrast, L2 learners exhibited a slowdown only when a singular noun was paired with a plural picture, and no effect when a plural noun was paired with a singular picture, revealing a processing asymmetry. For Japanese learners, plural nouns did not consistently activate conceptual plurality. Originality: This is the first study to examine both directions of morphological-conceptual number mismatch in L2 processing, extending Jiang et al., which tested only the singular-noun/plural-picture mapping. Significance: These findings refine the Morphological Congruency Hypothesis by showing that incongruent L2 morphemes can lead to direction-specific processing weaknesses in learners’ conceptual mapping.