Behind the broad, swift market slide of 2018 is an underlying new reality: Roughly 85% of all trading is on autopilot—controlled by machines, models, or passive investing formulas, creating an unprecedented trading herd that moves in unison and blazingly fast. That market has grown up during the long bull run, and hasn’t until now been seriously tested by a prolonged downturn.

Source: Wall Street Journal December 26, 2018 00:56 UTC