It is also widely believed that immigrants take local workers’ jobs, with the assumption that only a fixed number of jobs exist to go around. This is nonsense. We heard similar scare stories when women began to enter the labor force in large numbers: many men thought that if women started working, there would be fewer jobs for them. In fact, of course, most women now work, as do most men.
--Philippe Legrain, Freakonomics blog, on false fears of immigrants