The unpredictable role of language distance in bilingual cognition: A systematic review from brain to behavior

It has been argued that bilingualism under certain conditions can confer adaptations to the human mind and brain. Among the possible moderators of such adaptations, language distance occupies a distinctly ambiguous role. Equally unclear is the directionality of the effect. On the one hand, monitoring language may become more effortful depending on how similar competing alternatives are, thus predicting more robust adaptations in similar language bilinguals. On the other hand, the pool of competing alternatives grows with fewer overlaps between languages, and this widespread competition may require increased effort, predicting more robust adaptations in distant language bilinguals. In this systematic review and Bayesian analysis, we find robust evidence for a distance effect in bilingualism, but mixed evidence concerning its directionality. We interpret the results through charting the extreme variability that exists across studies, highlighting the need for developing ecologically accepted metrics of what counts as similar in language processing.