It's a pretty well understood and accepted fact at this point. It's been a major talking point in economic and political circles for the past 5 - 10 years.
The sources below are from a quick Google search. They mostly reference income stagnation and are about the US. But income stagnation leads to wealth stagnation and there are similar trends world wide. Thomas Picketty's Capital in the 21st Century is largely about this issue, as is Robert Reich's documentary Inequality for All. There are those who challenge the conclusions drawn in both, but few seriously contest the data they point to.
The sources below are from a quick Google search. They mostly reference income stagnation and are about the US. But income stagnation leads to wealth stagnation and there are similar trends world wide. Thomas Picketty's Capital in the 21st Century is largely about this issue, as is Robert Reich's documentary Inequality for All. There are those who challenge the conclusions drawn in both, but few seriously contest the data they point to.
[1] http://www.epi.org/publication/charting-wage-stagnation/ [2] https://www.npr.org/sections/money/2014/10/02/349863761/40-y... [3] https://www.economist.com/blogs/freeexchange/2013/09/incomes [4] http://www.nytimes.com/2013/01/13/sunday-review/americas-pro...