Suvarna Alladi et al., "Bilingualism delays age at onset of dementia, independent of education and immigration status", Neurology 2013:
Objectives: The purpose of the study was to determine the association between bilingualism and age at onset of dementia and its subtypes, taking into account potential confounding factors.
Methods: Case records of 648 patients with dementia (391 of them bilingual) diagnosed in a specialist clinic were reviewed. The age at onset of first symptoms was compared between monolingual and bilingual groups. The influence of number of languages spoken, education, occupation, and other potentially interacting variables was examined.
Results: Overall, bilingual patients developed dementia 4.5 years later than the monolingual ones. A significant difference in age at onset was found across Alzheimer disease dementia as well as frontotemporal dementia and vascular dementia, and was also observed in illiterate patients. There was no additional benefit to speaking more than 2 languages. The bilingual effect on age at dementia onset was shown independently of other potential confounding factors such as education, sex, occupation, and urban vs rural dwelling of subjects.
Conclusions: This is the largest study so far documenting a delayed onset of dementia in bilingual patients and the first one to show it separately in different dementia subtypes. It is the first study reporting a bilingual advantage in those who are illiterate, suggesting that education is not a sufficient explanation for the observed difference. The findings are interpreted in the context of the bilingual advantages in attention and executive functions.
The basic finding has been Out There for several years, e.g. Elen Bialystok et al., "Bilingualism, Aging, and Cognitive Control: Evidence From the Simon Task", Psychology and Aging 2004; Ellen Bialystok et al., "Bilingualism as a protection against the onset of symptoms of dementia", Neuropsychologia 2007. Previous LL coverage includes "What bilinguals tell us about mind and brain", 2/19/2011; "The bilingual advantage", 6/1/2011.
This new study is especially important for the reasons explained in this passage:
These questions can be addressed by studying populations in which bilingualism forms part of everyday life of the autochthonous population. Such a situation exists in India, a country characterized by an exceptional linguistic diversity. Several aspects of Indian bilingualism are important in the context of this study. First, bilingualism does not tend to be associated with immigration. Languages are usually acquired simultaneously and used in parallel and language switching is very common. Furthermore, bilingualism in India is contact-based and motivated by socialization processes and is therefore found even among those who are illiterate. Based on this unique social and linguistic setting, we aimed to study the association between bilingualism and age at dementia and its subtypes, taking into account potential confounding factors.
In the subject population for previous studies, bilingualism has been associated with immigration, so that bilinguals and monolinguals generally have different life histories, cultural backgrounds, and even genetics. This raises the concern that effects attributed to bilingualism might actually be the result of a correlated factor, such as childhood diet or whatever.
In the Indian study, bilingualism has quite different (and rather varied) socio-cultural associations:
In Hyderabad, the majority of the population can be considered as bilingual and many speak 3 or even more languages. Telugu is spoken by the majority group who are primarily Hindus, whereas the language of a minority group of Muslims is Dakkhini. English is gradually acquiring more functional roles in education, administration, and media. In addition, Hindi is spoken as the official national language and is taught at school level. Thus, most people in Hyderabad are exposed to Telugu and Dakkhini in informal contexts and Hindi and English in formal contexts. The patterns of language use in Hyderabad have been systematically studied and are well documented.
It's reassuring to see that the apparent delay in symptom onset due to bilingualism is essentially identical to those found in Bialystok et al. 2007:
On comparing bilingual with monolingual cohorts,bilinguals were found to be 4.5 older at the time ofoccurrence of the first symptoms of dementia: 65.6 years in bilinguals as opposed to 61.1 years in monolinguals.