The movement of human beings within and between countries is at an all-time high. Students, asylum seekers, businesspeople, and migrants move around the globe. Some countries have their natural population annually doubled or trebled by tourists. In others, particularly the Arab Gulf countries, foreign workers make up three quarters of the semipermanent population. Many universities have as many as a third of their students classified as foreign students.

There are half a million international students in America alone. In all, more than 200 million people live in a different country from their land of birth. People move for many reasons. There is a whole raft of pull and push factors involved in the difficult and complex decision to emigrate or even to experience an extended sojourn. ...