The Relationship of Bilingualism Compared to Monolingualism to the Risk of Cognitive Decline or Dementia: A Systematic Review and Meta-Analysis.

BACKGROUND Bilingualism may contribute to cognitive reserve, protect against cognitive decline, and delay the onset of dementia. OBJECTIVE We systematically reviewed evidence about the effect of bilingualism on subsequent cognitive decline or dementia. METHODS We searched electronic databases and references for longitudinal studies comparing cognitive decline in people who were bilingual with those who were monolingual and evaluated study quality. We conducted meta-analyses using random effects models to calculate pooled odds ratio of incident dementia. RESULTS We included 13/1,156 eligible articles. Meta-analysis of prospective studies of the effects of bilingualism on future dementia gave a combined Odds Ratio of dementia of 0.96 (95% CI 0.74-1.23) in bilingual participants (n = 5,527) compared to monolinguals. Most retrospective studies found that bilingual people were reported to develop symptoms of cognitive decline at a later age than monolingual participants. CONCLUSION We did not find that bilingualism protects from cognitive decline or dementia from prospective studies. Retrospective studies are more prone to confounding by education, or cultural differences in presentation to dementia services and are therefore not suited to establishing causative links between risk factors and outcomes.