This paper explores the code-switching behavior of second language (L2) bilinguals as a lens into the development of their L2 linguistic systems. Specifically, it investigates the acceptability judgments of L1-English L2-Spanish bilinguals on intra-sentential code-switching, comparing those judgments to a group of Spanish–English bilinguals who acquired both languages as an L1. The particular issues of proficiency and bilingual language behavior are analyzed, testing whether either factor has an effect on L2 code-switching intuitions. The results suggest that both proficiency and bilingual language behavior are relevant. L2 bilinguals with an intermediate/advanced proficiency level of Spanish were more likely to align with 2L1 bilinguals with regard to code-switching judgments, as were L2 bilinguals who reported prior experience with language mixing. L2 bilinguals with lower proficiency in Spanish, as well as those who reported never engaging in code-switching, however, were more likely to diverge from the 2L1 bilinguals in their judgments.
Among methodological concerns specific to code-switching (CS) research is the design of the target stimuli used in experiments with an acceptability judgment task. We argue here that research which makes use of CS data of this type must also incorporate monolingual stimuli into the experimental design, specifically monolingual stimuli judged by the same bilingual participants who judge the code-switched stimuli. We do so by reviewing two sets of experimental CS data we collected and exploring the role that monolingual stimuli can play in the analysis of that data. In each experiment, an analysis based solely on acceptability judgments of the CS stimuli leads to one interpretation, while incorporation of results from the monolingual stimuli leads to a distinct interpretation. We show that it is the interpretation integrating the monolingual acceptability judgments which is more valid, thereby arguing for the value of monolingual stimuli in design and analysis.