Defining bilingualism as a continuum Some tools and consequences for the study of bilingual mind and brain effects

The measurement of bilingualism is complex but essential for ( psycho)linguistic, cognitive and neural research. In addition to the conventional group comparison approach, namely comparing monolinguals to bilinguals, recent research has adopted continuous or multi-dimensional approaches. The aim of this chapter is to review a series of tools, including self-report usage and proficiency measures used in the literature. These tools were used with samples across the lifespan in North America and Europe. We further examine the aspects of bilingualism extracted from these tools. Finally, we outline the limitations of existing bilingual measurement tools and the need for researchers to justify the choice of each tool. Importantly, we advocate for taking a more nuanced understanding of bilingualism beyond individual usage and consider this experience as fluid, continuous rather than categorical.