Trends in economic mobility are difficult to prove because mobility itself is a long-term phenomenon. But for example, college tuition has increased dramatically, and a college education is one of the main ways to attain upward mobility.[1] Poor people are not able to enter profitable jobs (e.g. medicine, management, programming) because it requires training they cannot obtain, so they are instead stuck in e.g. minimum-wage retail work.

Economic inequality is easier to measure, and definitely correlates with (lack of) mobility.[2] Higher inequality means that there is a greater divide between the poor and the rich, so that it is more difficult to become rich if you are poor. Here, the statistics are clear: essentially all measures of inequality are increasing in the United States.[3]

America in the 1940s was mostly middle class: blue-collar jobs provided a livable wage, and income inequality was much lower.[4] In the past 50 years, middle-class wages have barely increased,[5] while college graduates[6] and CEOs earn much more.

[1] http://www.brookings.edu/research/reports/2013/06/13-facts-h...

[2] https://en.wikipedia.org/wiki/Socio-economic_mobility_in_the...

[3] http://www.newyorker.com/rational-irrationality/american-ine...

[4] https://en.wikipedia.org/wiki/Income_inequality_in_the_Unite...

[5] http://www.pewresearch.org/fact-tank/2014/10/09/for-most-wor...

[6] http://www.census.gov/library/infographics/sci_eng_majors.ht...