These studies always miss the obvious cultural point to me, [1] which is that knowing more than one language usually means you deeply understand more than one culture. This by default makes one a bit more capable of nuance, seeing other perspectives, etc. Languages are not just interchangeable collections of words, but are whole worldviews. Language in this sense is a kind of knowledge and not a different brain state, akin to reading books about history to understand a conflict better.

1. Maybe that’s not their fault, as they are ostensibly interested only in the biology. But it still seems like a major hole when discussing the benefits of being bilingual.