Nearly seventy years after its creation, the S&P 500 may be fit for purpose, but it is clearly no longer the narrow one of the 1950s. While the S&P was initially a tool for measurement and understanding, it has over time become a central actor in shaping investor behavior. The widespread use of S&P-based products now actively influences the market the index was intended merely to observe. Though not a true observer paradox in the quantum physics sense, the feedback loop between measurement an...
