Michael Clemens from the Centre for Global Development on immigration [emphasis mine]:
The research we have shows that immigration has had a positive effect on economic growth in Europe overall. This remains true in economists’ most sophisticated forecasts for the future. Christian Lutz and Ingo Wolter forecast a positive effect of immigration on German economic growth. Katerina Lisenkova and Miguel Sanchez forecast a positive effect of immigration on UK economic growth. And so on.
I would go as far as to say that this is a consensus opinion among economists. That is saying a lot, because economists are known for putting caveats on everything. But all the serious evidence we have points to large gains in overall economic activity from reduced barriers to labor mobility. Ninety-six percent of American labor economists agree that the economic benefits of US immigration exceed the losses.
That is essentially unanimity. While a handful of economists make vague claims of economic harm from immigration, they generally have not done any peer-reviewed economic research to support that claim, and their views should be regarded as political opinions rather than reflecting economic expertise.
Of course, speed matters. There are many reasons to expect the impact of a million immigrants to depend on whether they arrive over three years or over 20 years. This is largely absent from public debate, which tends to focus instead on absolutes like “stop them all” or “let them all in immediately.”
A more nuanced debate would begin from the solid consensus of serious economic research that there are large overall economic benefits, and discuss how to transition in order to capture those benefits. Economic development in poor countries is associated with more emigration—not less—for the same reasons that you’re more likely to see people from an outlying neighborhood living and working in an upscale part of your town-center when that outlying neighborhood gets richer. One of the great policy challenges of the 21st century is how to build policies that translate mobility into economic benefit, rather than building naval blockades and mass-detention camps.
I have recently been struggling to come up with a single policy/movement that would have a greater impact than opening borders (even gradually).