High Wages not Walls
People who decide to leave their country and test their luck elsewhere are usually no random sample of a population. In his 1987 paper “Self-Selection and the Earnings of Immigrants” (American Economic Review 77, pp. 531-553), Harvard Political Scientist George J. Borjas discusses the so-called self-selection of migrants. As of 1987, the standard view among migration economists was that migrants, at least those who came to the United States, belonged to the “upper tails” of the income distributions in their home countries.