I believe that's almost completely false. My wife was raised in a country where one language was spoken in the house, but she was taught at school in a complet

I believe that's almost completely false. My wife was raised in a country where one language was spoken in the house, but she was taught at school in a completely different language. Absolutely nothing to do with being middle class or having a higher upbringing. It was mandated by the State that she be taught in one language, but they couldn't force a local population to completely change their mother tongue in the home and in private. There are many other situations I can imagine, and know of first hand, where people of all socio-economic levels speak more than one language. The only time I can think bilingualism may be such a proxy is in a largely mono-linguistic culture.