Does Bilingualism Delay the Development of Dementia?

Paper	Method	Relevant findings
Bialystok, Craik, & Freedman (2007)	Participants: 132 with probable AD; 52 with other dementias. Recruited in Canada.	Symptom onset significantly later in bilinguals, even after controlling for demographics. First appointment at memory clinic was also significantly later in bilinguals.
	Definition of bilingualism: Bilinguals had “spent the majority of their lives, at least since early adulthood, regularly using at least two languages”. Monolinguals spoke only English, whilst bilinguals spoke English and at least one other language (e.g., Polish).
Craik, Bialystok, & Freedman (2010).	Participants: 211 participants with probable AD. Recruited in Canada.	Age of symptom onset and diagnosis significantly later in bilinguals.
	Definition of bilingualism: Same definition as Bialystok et al. (2007). Monolinguals spoke English. Bilinguals spoke English and at least one other language (e.g., Yiddish).	No significant effect of immigration status. No interaction between immigration status and language group on age of onset of symptoms or age of diagnosis. This suggests bilingualism delays onset and diagnosis regardless of immigration status.
Chertkow et al. (2010).	Participants: 632 participants with probable AD. Recruited in Canada.	Multilinguals had later age of diagnosis and symptom onset than monolinguals. No significant differences between bilinguals and monolinguals.
	Definition of bilingualism: Participants were monolingual (spoke one language), bilingual (spoke two languages), or multilingual (spoke three or more languages). Definition for bilingualism taken from Bialystok et al. (2007).	In non-immigrants, English-French bilinguals diagnosed significantly earlier than monolinguals. However, non-immigrant multilinguals diagnosed significantly later than non-immigrant monolinguals.
		In immigrants, bilinguals diagnosed significantly later than monolinguals.
Gollan, Salmon, Montoya, & Galasko (2011).	Participants: 44 bilinguals with probable AD. Recruited in the USA.	Overall, higher degree of bilingualism (assessed by BNT) was associated with a later diagnosis of AD.
	Definition of bilingualism: Participants spoke both Spanish and English. Degree of bilingualism assessed using the Boston Naming Test (BNT) and self-report.	Participants separated into low and high education group. Higher degree of bilingualism only protected against dementia in low education group.
Alladi et al. (2013).	Participants: 648 records of patients with dementia recruited in India. All participants were born and raised in India. Included individuals who were illiterate.	Bilinguals developed dementia significantly later than monolinguals, even after controlling for demographic factors. No additional benefit of speaking more than two languages.
	Definition of bilingualism: Participants were classified as monolingual (spoke one language) or bilingual (spoke two or more languages). Bilinguals were defined as individuals “with an ability to meet the communicative demands of the self, and the society in their normal functioning in two or more languages in their interaction with the other speakers of any or all of these languages” (Mohanty, 1994). Participants spoke a variety of languages, including Telugu, Hindi, and English.	Illiterate bilinguals developed dementia significantly later than illiterate monolinguals, suggesting effects are not due to education.
Zahodne, Schofield, Farrell, Stern & Manly (2014).	Participants: 1,067 Hispanic immigrants recruited in the USA. Records of 228 participants who developed dementia analysed.	Higher self-reported bilingualism associated with lower chance of dementia diagnosis, but no significant associations after controlling for demographic factors.
	Definition of bilingualism: Monolinguals spoke Spanish and bilinguals spoke Spanish and English. Participants were asked how well they speak English. If they answered “not at all”, they were classed as monolingual. If they answered “not well”, “well” or “very well”, they were classed as bilingual. A subset of participants completed the English-language Wide Range Achievement Test Version 3 (WRAT-3) to ensure self-report was valid.	Similarly, performance on the WRAT-3 did not independently predict rates of dementia diagnosis after demographic variables controlled for.
Lawton, Gasquoine, & Weimer (2015).	Participants: 1,789 Hispanic-Americans recruited in the USA. Case records of 55 diagnosed with probable AD and 26 with vascular dementia examined.	Proportion of monolinguals and bilinguals diagnosed with AD and vascular dementia did not significantly differ.
	Definition of bilingualism: Participants spoke Spanish and/or English. To assess language status, participants were asked “Do you speak Spanish?” and “Do you speak English”. If participants answered “not at all” or “not very often” about one of the languages, they were classified as being monolingual. If they answered “very often” or “almost always” to both, they were classified as being bilingual.	Bilinguals diagnosed later than monolinguals, though difference non-significant. Difference remained non-significant when immigrants and non-immigrants examined separately.
Woumans et al. (2015).	Participants: 134 participants with probable AD recruited in Belgium. All native Belgians.	Bilinguals experienced first symptoms significantly later and diagnosed significantly later, after controlling for demographics.
	Definition of bilingualism: Participants were asked how many languages they had experience using. They were then asked how proficient they were in each language and how often they used it. Participants were categorised as being bilingual if they estimated that they had good proficiency in speaking, reading, writing and listening in two or more languages, and used them both at least weekly. Monolinguals spoke Dutch or French. Bilinguals spoke a variety of languages, including Dutch, French, Spanish, German, and English.
Yeung, St John, Menec & Tyas (2014)	Participants: 1,616 older adults recruited in Canada.	No significant association between speaking more than one language and risk of dementia diagnosis.
	Definition of bilingualism: Self-report. Individuals were classified as a monolingual English speaker, a bilingual with English as a first language, or a bilingual with English as a second language. All participants spoke English. Bilinguals spoke English and another language (e.g., Ukrainian, German, or French).