Japanese-speaking learners of English are known to accept ungrammatical passive sentences such as *I was slept by my parents (e.g., Izumi & Lakshmanan, 1998). While this error has often been attributed to the influence of Japanese indirect passives, Takeda and Hokari (2023) demonstrate that causative passives also contribute to this phenomenon. This paper presents the results of a preliminary experiment designed to investigate this possibility further, addressing some confounding factors in Takeda and Hokari. Data collected from 17 Japanese-speaking learners of English and 19 native English speakers confirmed that the effects of causative passives are both more prevalent and persistent, as observed by Takeda and Hokari. Additionally, the results revealed that causative and indirect passive effects can coexist within individual learner grammars.
