And the University of Phoenix, America's largest private education provider, has sharply increased enrolments, from 384,000 to 455,000 in the past year.

In the United States, hundreds of millions of people have sought out new cheap housing in "new cities" and have been happy to settle in such places as Tampa, Dallas, las Vegas and Phoenix.

Susana Martinez-Conde is director of the Laboratory of Visual Neuroscience at the Barrow Neurological Institute in Phoenix.