The most significant early practical innovation to emerge from financial economics was the index fund. If, as financial economics suggested, managers' stock selections failed systematically to outperform broad market indices such as the S&P 500, then why not simply invest in the stocks that made up the index in such a way that the performance of one's portfolio would automatically track the level of the index? Such an index fund, its proponents suggested, would perform as well as the portfolios chosen by traditional managers, and it would not be hampered by the high fees those managers charged.

Donald MacKenzie, An Engine, Not a Camera, loc. 134