Iconicity does not always facilitate complex sentence comprehension: Behavioural and pupillometric evidence from Mandarin-English bilingual adults

English adverbial sentences allow flexible clause ordering: main-adverbial and adverbial-main, and their comprehension by English monolingual speakers is influenced by iconicity. Iconic sentences, whose clause order reflects the chronological sequence of real-world events, are more easily comprehended (“We ate dinner before we went to see a movie”; “After we ate dinner, we went to see a movie”) than non-iconic sentences where the order of the clauses does not match the order of events in the real world (“Before we went to see a movie, we ate dinner”; “We went to see a movie after we ate dinner”). However, Mandarin speakers strongly prefer the adverbial-main order in spontaneous Mandarin speech. This paper examines whether iconicity represents a generally preferred strategy for interpreting English adverbial sentences, or whether a speaker’s first language affects processing. In two studies, we tested the comprehension of English before- and after-sentences by Mandarin-English bilingual and English monolingual adults, using online pupillometry with offline accuracy in a sentence-picture matching task. We found that bilinguals, like monolinguals, comprehended after-sentences in the iconic adverbial-main order (also the preferred order in Mandarin) more accurately and with less effort than in the reverse order. However, unlike monolinguals, bilinguals did not comprehend before-sentences better in either the iconic main-adverbial order or the preferred adverbial-main order for Mandarin. Our results suggest that iconicity is not universal; instead, language-specific usage patterns, such as preferred clause order, compete with iconicity to influence processing of adverbial sentences.