Many people in poor places try to migrate to richer places — and so it has always been. But some poor places become rich places and immigration flows change as a result. There are many examples of “rathole” countries changing policies and becoming rich, and relatively rich countries becoming ratholes as a result of corruption and/or socialism (e.g., Venezuela). If a country has an open-border policy as did the U.S. before the 1880s, then large numbers of poor people migrate to the richer country, which, at some point, causes resentment for both good and bad reasons among the native-born population....