A result that can only be surprising in places where monolingualism is the norm, like the US, and knowledge of other languages (outside of those acquired from one’s family) is taken as a sign of erudition. If you’ve ever spent time in countries like Switzerland, you’d know their are plenty of people who speak 2 or more languages but are otherwise of average intelligence.
Switzerland is a bad example, because it's socially and multilingual.
Whereas if one looks at basically monolingual societies (e.g. most of the countries in Europe) one find that not only is not everyone bilingual (and no, "everyone" does not speak English) -- multilingualism definitely correlates with class and education.
Why would Switzerland be a bad example in this context? The point of the study is that bilingualism does not afford cognitive advantage, so you have to consider countries where bilingualism is not associated with the factors you cite to see if this bears out. Pointing at other countries that function in the same way as the US in regard to a dominant language would just demonstrate more of the same as is found there.
It's understandably hard to get actual numbers on the proportion of bilingual people.
There is a semi-random 50% number thrown around, but just to take the two biggest countries, there's no way we get accurate data on people in India and China given their size and lack of reporting of the inner regions. In China in particular, no one will boast about being fluent in a non-official language in a survey.
I think the same thing happen at smaller scale in european countries. There will be one official language, and whether people are bilingual or not won't be assessed more than striclty needed. The only study I saw were for educational purpose for small kids, out of that there will be no specific tracking of a Georgian immigrant starting a family in Germany, or a Spanish businessman being fluent in Italian because of his girlfriend.
It's understandably hard to get actual numbers on the proportion of bilingual people.
It's not "easy" to get the data but it's a well-studied topic. As attended to by people who presumably have thought about the matter and know what they're doing.
So I'll take their number (at around 60 percent for multilingualism) at face value. Seems to correlate with observations from traveling (through which one rapidly makes the amazing discovery that no, not "everyone" speaks English or is otherwise bilingual).
I kinda became sceptic of "trust us, it's ok" assumptions in many fields. I trust experts, but I also trust them to give insights on how they deal with the complexity, even if we don't look at the actual numbers etc.
For instance on this specific study all the research is based on Cambridge Brain Science/Creyos, and that gives me pause on how much the results are reliable (they are still valuable, way better than nothing, but might have huge flaws as well)
In that respect I'd like to know how they deal with "the gov might deport us in reeducation camp if we're too loud about our original culture" kind of situation, and wouldn't be surprised if the answer was "we pull numbers out of a magic hat", as it has been in many very prominent studies.
