From Group Comparisons to Individual Differences: A Researcher’s Manual to Test and Validate Models of Bilingual Variability

Recent years have seen a proliferation of models that attempt to explain the intricate relationships between bilingualism traits and social, cognitive, and neurolinguistic outcomes. The study of bilingualism has undergone significant conceptual growth, moving away from a binary, unitary perspective toward one that recognizes its diverse and dynamic nature. However, while theoretical models of bilingualism have evolved to incorporate modularity, contextual factors, and individual variability, methodological practices have not always kept pace. The gap between modern conceptualizations and empirical designs has hindered the field’s ability to generate incremental and replicable findings. This paper provides a methodological and practical manual to introduce interested researchers to individual differences theory and techniques. We present several approaches, statistical techniques, examples, and external resources to study individual differences in bilingualism with the goal of encouraging researchers to integrate them in their theoretical models and empirical studies in a cohesive and sound way.