It is fundamentally unjust to limit immigration beyond what is necessary for public safety.
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I also oppose all forms of birthright citizenship. Ius Solis is only marginally less abhorrent than Ius Sanguinis. I don't think anyone should ever be granted citizenship without informed consent. Citizenship should be an actual signed contract, not some nebulous implied social contract.
The problem with this is that the modern democratic political institutions in place can't work within that model. It's no accident that nationalism, the idea of passports, and representative democracy all developed together...
I'm not saying that you cannot come up with alternative institutions. There are alternatives. But I think that when defending such changes you should point out which alternatives you'd have go with them.
The US should have a population that's rising at a rate sufficient to prevent demographic problems with pensions and other aging issues down the road. That isn't going to happen with domestic birth rates.
That's just a bogeyman which seems to get invoked in every discussion about migrations. I have yet to see a single shred of evidence of such a real "demographic problem" happening in the modern world. Doomsayers have been talking for decades, and there's none to see. Higher productivity alone has negated it so far, just as substitution has negated the doomsayers of the Club of Rome in their predictions about resource depletion.
We're not seeing over the world any lack of able-bodied people to support retirees. We're seeing
rising youth unemployment all over the world!
But we also need the agricultural laborers, because we can't replace those out of citizens since the pay is so low and the work so miserable.
Doesn't that phase give you any idea about
what else you could change? If the work is "miserable" (I suppose you mean hard),
it is to be expected that the pay should be high. And it there were no takers at those wages, the pay would have to rise, and product prices along with it. Either pay up or import, what you would not be doing was to do without the food. Do not say that you need immigrants because the pay is so low, say that you need immigrants
so that the pay can remain so low.
I support the liberalisation of global trade: the free movement of goods, capital, labour and services. When we allow goods, capital, labour and services to move to where they are most productive, everybody wins. As has been said up-thread, that's how capitalism works. Any argument against opening borders to the free movement of labour is an argument against capitalism itself.
Ricardian economics applied to population! As if most people are eager to pack up and move for the sake of "productivity". Across national borders! At least Ricardo only wanted the workers to reproduce cheaper by letting them eat cheaper corn, not to become another trade commodity - he still saw them as slowly-changing factors of production. Perhaps it was because back then the era of workers as actual movable trade commodities (slavery) was just coming to an end. But yes, I can see how
that may be the pinnacle of an ideal capitalism... everybody won when the savages were brought from the jungle to the plantation, too. Even the savages,
because they became more productive. How grateful they must have been to their masters for their improvement!
My point is: this just never happens fast enough to work, economically, when people are doing it of their own free will and not coerced. When the industry in the northern UK was killed, how long did those regions kept having high unemployment? Why didn't the people all move away? It's a
slow process, even where it happens, where the personal barriers to movement (never mind borders or bureaucracy or culture) are low. You can make it faster, of course: just make people move or starve! But how far are you willing to go for the sake of productivity? "Everybody wins" if people are pushed around? Really? It will depend on whom you ask, and perhaps even when.
Beware of chasing ideal ideas which are "good for everyone" - the path to Hell is paved with (seemingly) good intentions, they say. I'm not trying to defend any particular position on transnational migrations, only pointing out that there is no "good for everyone" scenario, there are always winners and losers to consider.