This topic misses the forest for the trees. Even if there is a cognitive difference in people who speak multiple languages, the magnitude of the advantage/disadvantage pales in comparison to the far more impactful difference in the understanding of the world that proficiency in another language gives you. If you're a monolingual American born English speaker and you learn Chinese and go live in China for a year, you'…
> If being bilingual does have benefits over and above the _broader social, employment, and lifestyle gains_ that are available to speakers of a second language, then it should manifest as a cognitive advantage in the general population of bilinguals.