What evidence are you referring to? I have recently heard and read the opposite. See, for example: http://m.thisamericanlife.org/radio-archives/episode/562/the-

What evidence are you referring to? I have recently heard and read the opposite. See, for example: http://m.thisamericanlife.org/radio-archives/episode/562/the-problem-we-all-live-with http://m.thisamericanlife.org/radio-archives/episode/562/the... and http://www.nytimes.com/2015/05/05/upshot/why-the-new-research-on-mobility-matters-an-economists-view.html http://www.nytimes.com/2015/05/05/upshot/why-the-new-researc...