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Websites
Wikipedia: social mobility https://en.wikipedia.org/wiki/Social_mobility (pay attention to the list of "See Also" topics)
Brookings Institute http://www.brookings.edu/ Type "mobility" into the search box
Pew Charitable Trust http://www.pewtrusts.org/en/archived-projects/economic-mobility-project
http://www.pewtrusts.org/en/projects/financial-security-and-mobility
Articles
Pinsker, Joe. "America Is Even Less Socially Mobile than Economists Thought." The Atlantic. Atlantic Media Company, 23 July 2015. Web. 08 Feb. 2016.
<http://www.theatlantic.com/business/archive/2015/07/america-social-mobility-parents-income/399311/>.
"Mobility, Measured." The Economist. The Economist Newspaper Limited, 1 Feb. 2014. Web. 8 Feb. 2016.
Videos
CNN "How Socially Mobile is America?" http://globalpublicsquare.blogs.cnn.com/2013/08/04/how-socially-mobile-is-america/