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 meta-analysis 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.
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