As to your first point; there are many counterexamples where the people that remain in their country are as multi-lingual on average as the people who emigrate.

As to your first point; there are many counterexamples where the people that remain in their country are as multi-lingual on average as the people who emigrate. I would venture to say that level of ambition, education, opportunity & safety are more the factors influencing who would choose to emigrate and who wouldn't (or for whom the process would be administratively facilitated, since countries usually have profiles of desired immigrants), so I don't think its possible to infer a direct relation between language and intelligence by looking at skilled immigrants.