We often read of the benefits of the bilingualism – notably to delaying dementia. Research is confirming this – but only in particular instances. The difficulty surrounds defining bilingualism or, perhaps more accurately, defining different types of bilingualism.
One issue is the distance between the two languages involved. This language tree is a particlarly good demonstration of how close – both geographically and historically – different languages of Indo-European and Finno-Ugric origin are. The evidence is if someone speaks two languages which are very close together – say Norwegian and Swedish or Afrikaans and Dutch – then the benefits are extremely limited. The difference has to be at least as great as, for example, French to Spanish.
Another point is the context in which a language is spoken. If, for example, a Portuguese person moves to Brussels and uses French professionally but Portuguese at home, the benefit is limited – because the two languages are used in different spheres. Indeed, literally most people in the world are bi- or multi-lingual in such a way. A Berber, for example, may speak Berber at home, Arabic for trade and French for administration. These are three distinct languages but because their use never crosses over, the “bilingual benefit” is significantly reduced.
Then there is, of course, the difficulty with defining “speaking a language”. The ultimate benefit accrues to someone who grew up speaking two languages all the time and continues to use them – say, someone brought up in Belgium by one Dutch-speaking and one French-speaking parent who continues to use both languages professionally and in general daily life. It is more limited – though no doubt still present – for someone who grows up monolingual but learns and regularly uses another language fluently later in life. Then there is the issue of what is “fluency”? There is a range of levels – from being able to order to meal, to being able to “get by”, to being able to hold a conversation, to dreaming in the language; and then competence varies depending on how often the language was used and when it was last used (for example my own Spanish was somewhere between “hold a conversation” and “dreaming in the language” at the end of the half year I studied there; but it has dropped back a level, perhaps more, since).
Ultimately, research shows the benefit is specifically this: people who, when using a particular phrase, are “blocking out” another phrase from another language (because they are fluent in at least one other language) get the benefits referred to – this is something which keeps the brain exercised and thus has significant mental benefits including delaying dementia by an average five years. If, on the other hand, they are not “blocking out” another language – because that language is already similar, or because they would never be using it in that context, or because they’re not really fluent in the other language – will find those benefits reduced, to close to zero.
Whatever, this is a very interesting area for further research!