There is a book called "Capital in the Twenty-First Century" currently being discussed in the economics world that examines this phenomenon (wealth, however, not income). The tl;dr is that World War I, The Great Depression, and World War II distorted wealth accumulation, and the world is back on track to straight line wealth capture.
A more detailed analysis: http://www.nytimes.com/2014/01/29/opinion/capitalism-vs-demo...