Earlier quoted context omitted.
Can you elaborate? I'm pretty in favour of open borders as an immigrant myself but that statement could do with some supporting evidence.
It's not so much a question of evidence as basic math. Imagine a society with 100 people. A working adult can create 50 widgets per year, and there's 15 kids and 5 retirees. The economy produce 80 * 50 = 4000 widgets per year, which gives a per capita income of 40 widgets. You can have whatever tax, welfare, or income redistribution policies you like, but there's only 4000 widgets to go around. Great. Now let's say 5…
America has been quite successful with immigrants getting jobs. But in Europe some countries have for a while got a kind of immigration situation where unemployment among the migrants stays so high that they are actually worsening the ratio of workers to total population.