S&P 59

“The S&P 500’s total concentration, which we can measure using a Herfindahl-Hirschman Index (or HHI), is equivalent today to that of an equal-weighted, 59-stock portfolio. Ten years ago, the index was more than twice as diversified. We have never seen – over any 10-year period – a decline (or increase) in diversification of the magnitude we have just witnessed.“
Source: GMO.