Exit, Voice, and Loyalty (1970) is a notorious treatise in which Albert O. Hirschman hinges on a conceptual ultimatum that concerns any form of human grouping. The idea is that people have essentially two ways to express dissatisfaction: they can voice (complaining) or they can exit (withdrawing from the relationship). For example, the citizens of a country may respond to increasing political repression in two ways: protesting or emigrating.
Voting by feet is an extremely effective form of protest, especially these days. There are many interesting examples from the past. In the summer of 1990 Hungary opened up its borders with Austria and huge masses of East German vacationers drove their Trabants over the border, determined to stay. Interestingly, however, exit is not always an option; not even when it is encouraged. The right to leave one’s country cannot be exercised unless there is concomitant access to some other one. In 1930s, determined to rid Germany, the Nazis sought a way to expel the Jews; but in the face of their unability to do so, and encouraged by the unwillingness of western democracies to receive them, they resorted to mass murder.
Perhaps for this reason, the Universal Declaration of Human Rights and the Helsinki Agreement impose on liberal democracy the obligation to keep their doors open to a substantial extent so as to render not only immigration, but also emigration, always possible.