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

It has been argued that under certain conditions bilingualism 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, as juggling different languages may become more or less cognitively costly depending on how (dis)similar competing alternatives are. If different language pairings entail that a different degree of cognitive effort is needed to manage bilingualism, language distance asymmetries are predicted to differentially contribute to the robustness of bilingual adaptations. In this systematic review and Bayesian analysis, we find strong evidence for a distance effect in bilingualism, but mixed evidence concerning its directionality in terms of being more pronounced in similar vs. distant languages. We chart the extreme variability that exists across studies, highlighting the need for developing ecologically accepted metrics of what counts as similar in language processing.