We report a study that investigated executive functions in four groups of participants that varied in bilingual language experience, using a task that measured two theoretically motivated mechanisms of cognitive control (proactive and reactive control). Analyses of accuracy based on aggregated measures suggested an advantage in early highly proficient bilinguals over late passive bilinguals. However, when we factored in individual variability using mixed-model regression with a full random effect structure, we only found a marginal effect of language experience. Our results emphasise the importance of including individual variability when studying bilingualism, and highlight a fundamental consideration in research on the relation between language and attention – namely, the need for a theory-driven approach to measuring cognitive control through laboratory tasks.
