This is a short summary of recent research into how bilingualism strengthens the brain.
The details are a bit fuzzy, as is always the case in social science, but these results would not surprise me. In fact, it’s surprising to me that people ever thought bilingualism would be a hindrance to cognitive function, as opposed to an aid.
I think about bilingualism often as a math teacher. I try to consistently preach mathematical bilingualism, imploring students to train themselves to see problems both algebraically and geometrically. Being able to see a situation in two (or more) ways not only creates great flexibility in problem solving, but it strengthens the overall inter-connections between mathematical ideas.
It seems only natural to me that traditional bilingualism would do the same. Not only does it offer multiple filters for processing the world, but the development and maintenance of the cognitive systems is a real intellectual workout.