I have been writing about how well diversification works for a long time--with many articles on this topic published at Seeking Alpha. I published one of the first pieces about Target Date Folios (highly diversified portfolios designed for different stages of life) back in early 2008. I worked with FolioInvesting in the development of these strategies. Then the crash came. The Target Date Folios weathered that period (you can judge the results for yourself).
The last several years has seen a major rally in the S&P500. This period provides an outstanding environment to see whether diversification really adds value. I have a new post on this theme here.