Complexity researchers who study the behavior of stock markets may have identified a signal that precedes crashes.

They say the telltale sign is a measure of co-movement, or the likelihood of stocks to move in the same direction. When a market is healthy, co-movement is low. But in the months and years before a crash, co-movement seems to grow.

Regardless of whether stock prices go up or down or stay the same, they do so in tandem. People are copying each other, and a small nudge can send everyone in the same direction. The system appears primed for collapse.

“One of the most important things h...