Here’s one thing about the current volatility on Wall Street that stands out from previous periods of market turbulence: the tendency of individual stocks to move in the same direction.

Over the past three weeks, stocks moved together in waves, either with the vast majority moving up or the vast majority falling. Bespoke Investment Group describes such moves—when 400 or more of the S&P 500 issues move in the same direction—as “all or nothing days.”