By Alex Fradera

Some brains struck by pathology seem to stave off its effects thanks to a “cognitive reserve”: a superior use of mental resources that may be related to the way we use our brains over a lifetime, for instance through high levels of education or, possibly, learning a second language.

Bilingual people certainly seem to use their brains differently. For example, practice at switching languages has been associated with enhanced mental control. It’s even been claimed that being bilingual can stave off dementia by up to four or five years.

If true, this would have serious implications for public policy – learning a second language would be as much a desirable health behaviour as it is an educational or cultural one. But are the brain benefits of bilingualism real? The Journal of Alzheimer’s Disease has published a systematic review and meta-analysis to establish the strength of the evidence base.

The University College London research team, led by Naaheed Mukadam, surveyed hundreds of papers published up to November 2016, finding fourteen high-quality studies that measured dementia and/or mild cognitive impairment (a more subtle analogue and frequent precursor of dementia) as well as participants’ status as either mono-lingual or bilingual.

Eight of the studies drew their conclusions from patients who had attended clinics complaining of memory problems, and their results were in favour of bilingualism. In one study, the age of diagnosis for amnesic mild cognitive impairment was around four to five years later for bilingual patients compared to monolingual. Another five studies asked when people began noticing their symptoms, and these also found the similar four to five year delay for bilinguals. Two of the eight found no significant effects, but the preponderance of evidence here looks pretty good.

The trouble is, all of these papers were retrospective, and depended on when people decided to show up to a clinic, and/or their accurate dating of when their symptoms began. Complicating matters is the fact that bilingual people from minority ethnic backgrounds are known to seek help later for dementia, on average, probably for social and cultural reasons. Moreover, these studies rarely controlled for education, which tends to be higher for non-minority multi-linguals, and as education itself contributes to cognitive reserve, this may be another confound.

Better controlled studies that identify a sample of people free from dementia and then follow them in the future are easier to interpret. Mukadam’s team identified five such studies. These found that five years after recruitment there were no significant differences between bilingual and monolingual participants in terms of whether they had developed any memory problems or neurological diagnoses. A meta-analysis using the raw diagnosis numbers from each study – a total of just over 5,000 participants – confirmed that bilingualism does not stave off dementia.

But a weak evidence base doesn’t mean an effect isn’t real. Indeed, more recent research from a team of Italian researchers used neuroimaging techniques to make a more systematic case for the benefits of bilingualism, including that it is associated with greater neural connectivity.

This Italian work is too recent to feature in the new review, but it’s notable that it escapes some of the methodological problems discussed above. However, when I brought this to Naaheed Makadem (the lead author of the new review), she raised a few considerations: firstly, that the new neuroimaging study investigated people who were generally lifelong bilinguals, living in a bilingual context (the Italian-German borderlands of Italy), which affords more opportunities for lifelong mode-switching than would be found for cases of adult migration or deciding to pick up a new language; secondly, she points to other potential confounding factors, such as employment and social status (i.e. the apparent neural advantages of bilinguals might be related to their being more likely to be in employment and higher social status).

The multi-lingual mind may have important consequences, but it’s not clear that recommendations to learn a second language later in life will necessarily produce the protections that have been claimed. Makadem’s team conclude their work by suggesting that “public health policy should therefore remove recommendations regarding bilingualism as a strategy to delay dementia and instead concentrate on more generally reducing cognitive inactivity.”

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

—The impact of bilingualism on brain reserve and metabolic connectivity in Alzheimer’s dementia

Alex Fradera (@alexfradera) is Staff Writer at BPS Research Digest