I thought the usual claim was not “immigration increases total GDP” but “immigration increases per-capita GDP”. Random example: this paper which, full disclosure, I have not read but only looked at the abstract linked.
I’m not sure any measure of GDP (total or per capita) is a great way of assessing immigration. Consider the following scenarios:
A scenario like Phil’s: An immigrant moves from a poor country to a rich country. They somehow become less productive when they do this, and earn less than they did before. But their income is positive, so the total GDP of the rich country goes up.
A scenario showing the opposite problem: An immigrant moves from a poor country to a rich country. In the rich country they earn less than the average person there, but more than they did before. The immigrant is better off. The other inhabitants of the rich country are (in total) better off. But per capita GDP has gone down.
It seems to me that to avoid Simpson’s-Paradox-like confusion what we really want to know is: when some people migrate from country A to country B, what happens to (1) the total “GDP” of just the people who were already in country B and (2) the total “GDP” of just the people who moved from country A? We might also care about (3) the total “GDP” of those who remain in country B. My guess, FWIW, is that #1 and #2 both go up while #3 goes down.
I thought the usual claim was not “immigration increases total GDP” but “immigration increases per-capita GDP”. Random example: this paper which, full disclosure, I have not read but only looked at the abstract linked.
I’m not sure any measure of GDP (total or per capita) is a great way of assessing immigration. Consider the following scenarios:
A scenario like Phil’s: An immigrant moves from a poor country to a rich country. They somehow become less productive when they do this, and earn less than they did before. But their income is positive, so the total GDP of the rich country goes up.
A scenario showing the opposite problem: An immigrant moves from a poor country to a rich country. In the rich country they earn less than the average person there, but more than they did before. The immigrant is better off. The other inhabitants of the rich country are (in total) better off. But per capita GDP has gone down.
It seems to me that to avoid Simpson’s-Paradox-like confusion what we really want to know is: when some people migrate from country A to country B, what happens to (1) the total “GDP” of just the people who were already in country B and (2) the total “GDP” of just the people who moved from country A? We might also care about (3) the total “GDP” of those who remain in country B. My guess, FWIW, is that #1 and #2 both go up while #3 goes down.
We might also want to compute the sum of the GDP of A and B: does that person moving cause more net productivity growth in B than loss in A?