Does bilingualism protect against dementia? A meta-analysis
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THEORETICAL REVIEW
Does bilingualism protect against dementia? A meta-analysis John A. E. Anderson 1 & Kornelia Hawrylewicz 2 & John G. Grundy 3
# The Psychonomic Society, Inc. 2020
Abstract Evidence suggests that bilingualism may contribute to neuroplasticity and cognitive reserve, allowing individuals to resist cognitive decline associated with Alzheimer’s disease progression, although the idea remains controversial. Here, we argue that the reason for the discrepancy stems from conflating incidence rates of dementia and the age at which the symptoms first appear, as well as statistical and methodological issues in the study designs. To clarify the issues, we conducted a comprehensive metaanalysis on the available literature regarding bilingualism and Alzheimer’s disease, including both retrospective and prospective studies, as well as age of onset and incidence rates. Results revealed a moderate effect size for the protective effect of bilingualism on age of onset of symptoms of Alzheimer’s disease (Cohen’s d = 0.32), and weaker evidence that bilingualism prevents the occurrence of disease incidence itself (Cohen’s d = 0.10). Moreover, our results cannot be explained by SES, education, or publication bias. We conclude with a discussion on how bilingualism contributes to cognitive reserve and protects against Alzheimer’s disease and recommend that future studies report both age of onset as well as incidence rates when possible. Keywords Alzheimer’s disease . Bilingualism . Dementia . Meta-analysis
As adults age, cognitive functions decline due to loss of gray matter (Thompson et al., 2003), white matter (de Mooij, Henson, Waldorp, & Kievit, 2018; Ge et al., 2002), brain signal complexity (Dauwels, Vialatte, & Cichocki, 2010; Fernández et al., 2010), and functional connectivity between brain regions (Brier et al., 2012). These losses affect language (Weiler et al., 2014), memory (Nyberg, Lövdén, Riklund, Lindenberger, & Bäckman, 2012), motor control (Seidler et al., 2010), and higher-order executive processes (Borghesani et al., 2013) across the population. According to the World Health Organization (2019), age is the single strongest risk factor for dementia, and dementia currently affects more than 50 million people worldwide. This number is John A. E. Anderson and John G. Grundy denotes shared first authorship * John A. E. Anderson [email protected] * John G. Grundy [email protected] 1
Centre for Addiction and Mental Health, Kimel Imaging and Genetics Laboratory, Toronto, ON, Canada
2
Department of Psychology, Faculty of Health, York University, Toronto, ON, Canada
3
Department of Psychology, Iowa State University, Ames, IA, USA
projected to continue doubling approximately every 20 years (Prince, 2015; Prince et al., 2013) and in 2015 alone cost approximately US$818 billion (Wimo et al., 2017). By the time people reach 85 years or older, as many as 1 in 4 people are diagnosed with dementia (Prince, 2015; Prince et al., 2013). Given the numerous advances in the medical field leading to increased
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