|A bilingual Hebrew-Arabic classroom in Israel|
Science began investigating the question about a century ago, producing pretty different results from what we now consider true. In the early 20th century, raising children in multilingual environments was thought to be a primary cause of stunting and retardation, preventing proper cognitive and linguistic development because the child had to manage multiple vocabularies, and might get confused.
There is a grain of truth to this. Multilinguals have more limited vocabulary in each of their languages, usually because the languages might only occupy certain segments of their life - if someone uses one language at home and another at work, they might not know the term for "shampoo" in the second language, or "consumer demographics" in the first. There's also evidence that the need to suppress one language in favor of another while performing specific linguistic tasks can increase the mental load.
|Example: I don't have a clue what the Dutch word for seatbelt is, because I never need to drive when I'm in the Netherlands|
Currently, though, the view that multilingualism negatively affects general cognitive or linguistic capacity is not supported - the debate is rather over whether it has significant positive effects. The most common claims about positive effects of speaking multiple languages are that it enhances executive function and prevents cognitive aging, but both are subject to dispute.
It's a well-bolstered claim that bilingualism slows mental decline with age, specifically delaying the onset of Alzheimer's and dementia. The mechanisms are not well understood - it's been postulated that multilingualism enhances the "cognitive reserve" by increasing the number of neural connections, providing a more resilient neural infrastructure as some connections begin to decay. Evidence for the idea comes chiefly from medical records. A well-known study with a Toronto hospital by Ellen Bialystok came up with the figure that speakers of more than one language develop Alzheimers, on average, four and a half years later than monolinguals. As far as I could tell, claims about staving off mental disease are subject to relatively little controversy.
|Plaques associated with Alzheimer's prohibit neuron growth, damaging the resilience of neural networks|
Some researchers also say that having to handle multiple languages builds executive function, the ability to manage attention and ignore irrelevant stimuli. The idea is that the constant mental load of managing multiple languages exercises the capacity to suppress unneeded information. This proposition is more hotly contested, with detractors claiming that the tests for executive function are ineffective and inconsistent, and that publication bias has favored small, poorly executed studies that show positive results by random chance. There have been plenty of studies performed which don't demonstrate a significant advantage (though this is science, where absence of evidence doesn't necessarily mean evidence of absence.) The consensus being approached is that the cognitive benefits of multilingualism are nuanced, and difficult to agreeably determine until study methods are refined.
Of course, all of this ignores the social benefits people get from speaking multiple languages, and the main reasons people do it at all. It's unlikely that babies learning two or three languages just from their family, or the hundreds of millions of people studying English or other global languages in hopes of a better education or job, are waiting with bated breath to hear whether they also get marginally better stimulus control before they go ahead and become multilingual. Still, in a world where most people can carry on a conversation in two or three languages, it's nice to know if it makes our minds better too.
|In plenty of communities, knowledge of multiple languages is the norm.|
Bialystok et al.
Carlson and Meltzoff
Mechelli et al.