"In old age, bilingualism has a positive effect on the structure of the brain and cognitive ability"

Nevertheless, it is not a new finding that strong use of the brain builds up a so-called cognitive reserve. Isn't bilingualism just one of many ways to build up such a reserve?

Wagner: On a superficial level, you can actually say that all activities in early life - whether you play chess, learn an instrument or travel the world a lot - ensure that you have a higher level of performance in old age and that it therefore takes longer to develop dementia. But there has been a tremendous amount of research in recent years on the topic of cognitive reserve, and there has been a shift to looking at resilience to the pathological process.

What does that mean?

Wagner: It can be shown that the pathological process - i.e. the progression of the disease - does not affect a patient's cognitive abilities as much if they have more reserves. However, we did not actually investigate this in our study: We did not include any indicators of the pathological process.

Is the positive effect actually due to bilingualism itself? After all, people who speak two languages every day usually live in a situation that is more challenging in itself - for example, studying abroad or working in a foreign country.

Wagner: In epidemiology, this is called confounding by known or unknown third variables: In our example, bilingualism would be a result, but in reality it is only an indicator for something completely different. We have statistically controlled this for known third variables such as education. One can speculate for a long time about other variables.

Kuhn: But our pattern of results with stronger effects of early bilingualism rather speaks against the fact that unknown and uncontrolled third variables have distorted our results. Living in another country, for example, should still be stimulating at an older age - and then one would have expected equally strong effects with later multilingualism. Therefore, we are quite sure that bilingualism is indeed the factor responsible for the results - and not merely an indicator of something else.

In your study, you rely on data from the DELCODE study, in which around a thousand subjects are followed at the DZNE over many years. When you designed this study, did you already specifically plan to investigate bilingualism?

Wagner: A cohort study of this kind always has numerous goals; in DELCODE, the focus was, among other things, on risk and protective factors with regard to cognitive deterioration in old age. DELCODE has a questionnaire that we call "Lifetime of Experience". In this questionnaire, we ask the respondents about many different aspects of their lives, including multilingualism. So it is one aspect of many - but we clarify it in sufficient depth because we ask about how often a second language is used, at what age it was learned and so on.

Don't the results mean that migrants and refugees should be much less vulnerable to dementia?

Wagner: That is perhaps true under certain conditions, but migrant status is also associated with many other factors, from often lower socio-economic status to poorer housing conditions to social stress. Therefore, the question is difficult to answer. At any rate, no results can be derived from our study directly, since only people with a good knowledge of German participate in DELCODE - otherwise they would not be able to cope with the cognitive tests simply because of their language skills.

Is the topic of bilingualism now finished for you or are you continuing to research it?

Kuhn: We definitely want to come back to it - at a later point in time, when more data is available in the DELCODE study. Above all, we want to investigate whether bilingualism helps the subjects in the long run when it comes to slowing down the effects of pathological processes. We are currently collecting the data we need for this.

Your study has has attracted international attention: The New York Times reported, you gave an interview to the BBC - how do you explain the interest?

Wagner: Everything that has to do with protective factors against Alzheimer's arouses great curiosity and, of course, hope. Everyone wants to know what helps and what they can do themselves to protect themselves against dementia.

Were you able to convey good news in these interviews?

Wagner: Yes, our study on bilingualism is of course encouraging. But unfortunately I also have to say that anyone who rushes into a Spanish course after retirement and crams vocabulary in order to be armed against future dementia - that probably won't help them, at that point it's simply already too late. But continuing to speak languages learned earlier, and generally keeping mentally and physically active, is good in any case.