Since the beginning of this year, that correlation has jumped to 60% or even higher. On some three-day stretches it has even hit 90%.
That's because when prices of some key assets -- like oil -- fall dramatically, investors have a tendency to start selling across the board, without much thought. It's similar to what happened during the financial crisis or even the panic sell-off in early October.
"It's the pain trade," says Block. "Stock prices aren't just driven by fundamentals. They're driven by who owns what."
He explains that losses in one part of a portfolio often trigger selling in other parts out of fear or because people need to raise cash to cover a bad position and they have to dump what they can, even if they don't want to.