A tight prior I hold is that people want to live where they can reach the highest standard of living. This will generally happen where people have the most "economic freedom," the ability to choose what they want to do. This map of the world that accounts for net immigration roughly shows this to be the case. Fat regions have high rates of net immigration.
I was at first surprised at the size of the Middle East, but then here is the Heritage Foundation's Index of Economic Freedom. While no Middle East country is listed there as being "free", Saudi Arabia, the United Arab Emirates, and Bahrain are listed as being "mostly free."
HT to Marginal Revoluion where other cool maps are listed.