Conventionally, the professional interpreter speaks in the first-person singular. Research in corpus-based discourse analysis has reported shifts from this norm towards first-person plural becoming the most frequent pronoun shift in political institutional interpreting, possibly signalling interpreter 'alignment with the institution'. Nonetheless, few studies have teased apart the simultaneous constraints of social, cognitive, and linguistic factors on institutional interpreters' preference for the plural. The present research adopts the usage-based theory to consider the three types of explanations together. It extends recent multivariate methodologies based on this theory to analyse 2,438 first-person cases in parallel interpreting and comparable speech corpora. Following robust context analyses and cross-linguistic prosodic transcription, this study weighs the strengths of 33 associates regarding the three explanations through regression analyses. The results show that first-person shifts are better explained by chunking effects when interpreters process complex forms and referents in the source and target speeches, and when they process zero-subject source inputs. The institutional alignment explanation fails to account for the extensive grammaticalisation of plural constructions in the interpreted speech. When all the interactive and additive effects are considered together, institutional alignment or monofactorial paradigms have little explanatory power. This study concludes by highlighting the relevance of usage-based multifactorial designs to interpreting research.
