To talk about this in a useful way, you'd probably have to isolate 3-7 distinct demographics of immigrants...for example, refugee populations are very different in terms of incentives and outcomes when compared to populations who come seeking degrees, who in turn are very different from those who come for the purpose of wealth and a higher quality of life.

My family is in the "education / science" demographic - lots of grad students, post-docs, and professors come to the US because of the better quality of education / more research opportunities. The US natural sciences in general have very high immigrant populations at every level.