Why was immigration freer in 19th century USA?

In a blog post titled The Golden Age of Migration, Bryan Caplan notes that, despite its many flaws, the 19th century US was morally better than the current US in one important respect: open borders. Caplan quotes a passage from the book Exceptional People: How Migration Shaped Our World and Will Define Our Future and adds:

Imagine – there was a time when elite opinion and public policy took free immigration seriously. All democracy did to tarnish this political miracle was scapegoat Asian immigrants, while leaving the doors open to not just Europe, but Latin America as well. Whatever its flaws, the Gilded Age was truly the Golden Age of Immigration. Libertarians – and anyone who cares about the genuinely poor – should give credit where credit is due.

An important question is: what has changed since then, and why? There seem to be three kinds of reasons:

  1. Public and elite opinion about the wisdom/desirability of immigration restrictions has changed. See the then versus now page for some arguments that have been offered about the differences between the present and the past.
  2. The technological/financial feasibility of immigration restrictions has changed. In the 19th century, the US federal government was small, hence it lacked the financial and technological resources to enforce immigration restrictions. Passports had not been introduced. Now, with the exception of migration along the southern border, most forms of immigration into the United States can be controlled at relatively low direct cost.
  3. Public and elite opinion about the moral permissibility of immigration restrictions has changed. In the 19th century, however much people hated immigrants, shutting down immigration just wasn’t an option morally on the table, just as deportation to Africa is not a moral option to the problem of black crime in the United States today.

What is the right mix of reasons? My best guess: (1) is unlikely, because anti-foreign bias has been there since the dawn of history. Further, the problems created by immigrants, both real and perceived, haven’t shrunk that much since the 19th century. So, I suspect the reason is some sort of mix of (2) and (3), where they both feed into each other — if immigration restrictions aren’t technically feasible, then they aren’t a salient option morally either, and vice versa.

Any other ideas?

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