(Agreed about the immigration rationalization though)

Um, as far as immigration. You may have noticed that some countries are much nicer places to live then others, i.e., some have low crime and highly functioning economies and others are poor crime filled hell-holes. Why is that? Is it that something about being north of the Rio Grande magically makes people more productive and less prone to commit violent crimes? <\sarcasm>

The main reason is the people and culture of those countries. Thus if you import too many people from a different country, the pleasantness of the country to live will depend on the the nature of the new people. Notice that this argument assumes nothing about the role of nature versus nurture.