I think there is an elephant in the room here. There is no easy way to have a control subject.<p>Each person they test has a unique set of abilities and intelligence level.<p>My children are bilingual, and I know many other families that have children who are being brought up in a bilingual environment.<p>I have an example of a bilingual child that didn&#x27;t say anything almost at all until they were 2.5 years old. My daughter has language skills that are advanced for her age in comparison to her monolingual peers of the same age. But she is just bright (parental bias I know). My younger son appears to be improving even faster, but then he has a bilingual older sibling, so that isn&#x27;t a surprise.<p>The point is that it all depends on the person, as to how well they can adapt to a bilingual upbringing. They are so many factors involved, that it becomes impossible to categorically compare one person against another. Apples to oranges, every single time.