Overpassivization Revisited: Disentangling Syntax and Semantics in Causes of Errors

English passives and unaccusatives share similarities in both syntactic derivation and semantic properties of the subject argument. However, similarities in syntactic derivation have primarily been discussed in the literature as a core trigger of prevalent overpassivization errors with unaccusative verbs by second language learners of English. In this study, we conducted two experiments to investigate which cue, either syntactic similarity or semantic similarity, is active for incorrectly applying passive frames to unaccusative verbs. The first experiment assessed whether learners represent unaccusatives as sharing a syntactic derivation with passives. Based on the assumption that stress patterns reflect syntactic derivations, participants judged acceptability of stress patterns of intransitives and passives. The results did not provide conclusive evidence that L2 learners treat unaccusatives and passives as sharing a common syntactic derivation. Even the five most native-like learners showed residual optionality, and their divergent performance on passive versus intransitive constructions indicates that highly advanced learners may still lack fully distinct syntactic representations for the two types of intransitives. In the second experiment, we investigated whether learners instead rely on thematic information. The results show that learners consistently rated unaccusative subjects as less agentive than unergative subjects, indicating that they recognize the thematic-role similarity between unaccusatives and passives. Taken together, the findings suggest that semantic similarity between passives and unaccusatives, rather than syntactic identity, drives learners' overpassivization behavior.