Delighted to see Noah Smith introduce Bloomberg readers to the case for open borders.  Two reactions:

1. When Noah describes the economic benefits of immigration, he makes it sound redistributive: Our GDP goes up because we gained people, which presumably means the sending countries’ GDP goes down by the same amount because they lost people.  At least that’s the natural way to read this passage:

The way to get 4 percent growth is open-borders immigration policy.

Gross domestic product is simply the product of output per person and
the number of people. The more people in your country, the higher the
output. That’s why China, whose output per person is only about a
quarter of the U.S.’s, is now the largest economy on the planet. It just has more bodies. 

growth numbers you usually hear about in the news are total GDP growth
numbers, not per capita figures. To boost those numbers, get more
population. For example, when Great Britain conquered India, the GDP of
the British Empire went way up. If the U.S. really wanted to supercharge
its GDP numbers, it has a much better option than military conquest —
it could simply invite tons of immigrants to move here.

The point I’m confident Noah grasps, but fails to communicate: Due to vastly higher labor productivity in the U.S., U.S. GDP would rise more than sending countries’ GDPs would fall.*  Any chance you can write a followup explaining this, Noah?  Please please please.

2. Noah generously hands me some free publicity, but what he says isn’t quite right.

Exactly this sort of open borders immigration policy has received enthusiastic support
from a dedicated core of libertarian economists, notably Bryan Caplan
of George Mason University. These economists believe in relaxed
immigration rules not because they want higher GDP growth, but because
of principle — they view national borders themselves as an unacceptable
form of government intervention in the economy.

Actually, I believe in open borders for both reasons, and many others.  Yes, I have a moral presumption against regulation in general.  But regulations that impoverish billions are much worse than regulations slightly harm hundreds.

Noah continues:

The open borders
crusaders are so zealous that moderate supporters of increased
immigration, such as tech entrepreneur Vivek Wadhwa, are often the targets of their ire. University of Chicago economist John Cochrane has also voiced support for the open borders idea.

Here’s what I previously said about Vivek:

I’m glad the world has moderately pro-immigration thinkers like Vivek.  I
don’t think the heretic is worse than the infidel.  Even baby steps
towards open borders are steps in the right direction.

That’s zeal?  That’s ire?

* This is true even though U.S. per-capita GDP would go down!  Under open borders, both current residents and new arrivals would be richer.  The
composition of the population would however heavily shift toward low-skilled

Example: Suppose initially, a country has 100% high-skilled
natives earning $40,000 a year.  Low-skilled foreigners earn $1000 a
year in their home countries.  After open borders, the U.S. population
shifts to 50% high-skilled natives earning $50,000 a year, 50%
low-skilled foreigners earning $10,000 a year.  All individuals are
richer.  But U.S. per-capita GDP just fell from $40,000 (1*$40,000) to $30,000 (.5*$50,000 + .5*$10,000).