Here's some encouraging news for stock-market bears ...

Margin debt (red in the chart above) hit a new high in April.

Even after adjusting for inflation, margin debt is now higher than it was at the peak of the great bull market in 2000 and the echo bull market in 2007.

What is "margin debt"?

It's the amount of money stock investors have collectively borrowed via traditional margin accounts to fund stock purchases.

In a bull market, the growth of margin debt serves as a turbocharger that helps drive stock prices higher. 

As with a home mortgage, the more investors borrow, the more house or stock they can buy. So as margin debt grows, collective buying power grows. The borrowed money gets used to fund new stock purchases, which helps drives the prices of those stocks higher. The higher prices, in turn, allow traders to borrow more money to fund additional purchases. And so on.

It's a self-reinforcing cycle.

The trouble is that it's a self-reinforcing cycle on the way down, too.