>> It looks like every kind of screening of immigrants will deter the more desirable ones, as far as that determination is possible on their arrival at all.

I would say in the case of the US, just getting here requires a certain amount of motivation. Even the folks crossing the border from Mexico have to be rather motivated. Anyone coming from another country usually has to pay for transportation at the very least. When the country was founded, these challenges were much greater so I figure the US had the advantage of getting only immigrants above some level of motivation when it started - in addition to the other advantages it had.