This study aimed to investigate how Japanese learners of English as a foreign language, whose first language does not have obligatory morphological number marking, process conceptual plurality. The targeted structure was reciprocal verbs, which require conceptual plurality to interpret their meanings correctly. The results of a sentence completion task confirmed that participants could use reciprocal verbs reciprocally in English. In a self-paced reading experiment, participants read sentences with reciprocal verbs and those with optionally transitive verbs (e.g., while the king and the queen kissed/left the baby read the book in the bed). There was no reading time delay for reciprocal verbs but a delay for optionally transitive verbs. Therefore, the participants succeeded in processing second language conceptual plurality in the online sentence comprehension task.

This article addresses the pitfalls of performance analysis in investigating cognitive processing during second language (L2) learning. The problems that we discuss in this paper are twofold: (1) Assuming psychological variables to be ontological entities without meeting the criteria for ontological reality and (2) Inappropriateness of assessing abilities based on learner’s speaking or writing performance to investigate cognitive processes. By addressing these problems, we argue that some latent variables postulated by observing L2 performance do not exist in reality and emphasize the difficulty of interpreting cognitive mechanisms through performance analysis. We also enumerate some problems that arise from the epistemological perspectives of previous research practice (e.g., the bifurcation of contradictory hypotheses and their indeterminacy). Finally, two alternative approaches treating L2 performance are proposed. The implications of this line of discussion for future research are also discussed.

Although native speakers (NSs) of English make plural agreement in preverbal-subject sentences (e.g., A pen and eraser *is/are…), previous studies have demonstrated that they prefer singular – not plural – agreement between verbs and conjoined noun phrases (NPs) in expletive there constructions (e.g., there is/are a pen and an eraser…), showing efficiency-driven processing prioritization of agreement between nearest constituents. This paper assesses whether Japanese L2 learners of English (JLE) show this tendency. The results of two self-paced reading experiments together indicated that even though efficiency-driven processing was available to L2 learners, their use was unstable due to the repeated exposure to there are NPpl- and NPpl-type sentences during the task. It seems possible that repeated exposure triggered learners’ knowledge that conjoined NPs are always plural. Hence, it could conceivably be hypothesized that a learner’s specific knowledge intervenes the efficiency-driven processing strategy.