Limits on immigration leave trillion dollar bills on the sidewalk -- as Michael Clemens famously puts it. The idea is common sensical to poor people around the world, many of whom do what they can to enter the richer countries. It is also relevant to the much discussed "Mobility Myth". For the U.S. case, unless the data used to study mobility include those who have made it across the border, the analysis is incomplete -- and misleading.
Mobility studies necessarily pick a time window and follow people (or their families) over whatever span the data allow. This means that any who arrived within that time frame are left out; their forbears were not on hand to be included. The most upwardly mobile are systematically excluded from the analysis. This skews the results.
Raj Chetty and his co-authors follow U.S. parents and offspring over about 30 years. Anyone who arrived in the interim and stayed is left out. But these are plausibly the most upwardly mobile. How many arrived? This source says at least 20 million legal arrivals in that period. This source suggests many more.
Foreign born or not, they are all people. We should want them all to have the chance to move up. The more, the merrier. I wish a way could be found to let more people settle where they want to. In the interim, let's celebrate those who made it -- and who managed to improve their families' prospects.
They are, for the most part, good news that must be included against all of the hand-wringing over mobility.