"The shift from active to passive investing is a significant amplifier of future volatility. Active managers serve as a volatility buffer, willing to step in and buy undervalued stocks when the market is falling, and sell overvalued stocks when the market is rising too much. Remove that buffer, and there is no incremental seller to control overvaluation on the way up, and no incremental buyer to stop a crash on the way down.”

Volatility and the Alchemy of Risk, Reflexivity in the Shadows of Black Monday 1987, October 2017