From an editorial in Le Monde:

Recent research has shown that the reinforcement of French ghettos can be explained by the flight of lower and upper classes who do not want to take the chance at social mixing.

According to Eric Maurin, “The French ghetto is not so much a place of confrontation between the included and the excluded as a theater in which each group makes a virtue out of flight or sidestepping the group immediately below it.”

[Crappy translation is my own.]