Over the past seven decades, U.S. equity-market volatility has roughly doubled—from about 10% to 20%—and this increase is concentrated at the market level and at high frequencies (daily volatility up by ~130%, weekly by ~75%, monthly by ~40%). A new paper by Lars Lochstoer and Tyler Muir argues that this structural change is not driven by macroeconomic fundamentals or firm-level shocks but by the dramatic growth of index-level trading (futures, ETFs, index mutual funds, and extended trading hours). Using statistical investigations—the 1997 introduction of E‑mini S&P 500 futures and historical NYSE trading‑hour changes—the authors provide causal evidence that easier and larger trading of the market portfolio has raised aggregate volatility through higher trading volume and a shift toward systematic demand shocks.