Russ Roberts: The reason it's important, more generally, is because: If, to take an example, the United States had a lot of low-skill immigrants come into the country between 1980 and 2014--which, there's a decent number of. Obviously it's a bunch. But there's also some immigrants who came, which complicates it. But, if you had low-skilled-only immigrants who came, then what you would observe is--you that the average wage level in the United States could go down, but every person who was already here before the immigration are better off. And so, the average growth rate for the bottom 50% in that situation could be very misleading about the state of the economy. Could be just a composition effect.


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