Stats › Concentration and breadth › Index effect
Joining the S&P 500 was worth 7.4%, now about zero
The decline from the 2000s to the 2010s is 4.4 percentage points and statistically significant at p below 0.001. Source: Greenwood and Sammon, The Disappearing Index Effect.
Companies added to the S&P 500 once rose in price for no reason connected to their business, because index funds had to buy them. That gain peaked near seven percent in the 1990s and is now indistinguishable from zero. Indexing grew and the effect vanished.
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