Indicators of a market top began piling up way back in 2014, and since then most have gone on to unprecedented extremes. Every analyst has their favorite harbinger of financial doom, but the easiest to understand and the most tragic is probably margin debt.

This is money borrowed by (usually individual or “retail”) investors against their existing stocks to buy more stocks. Investors tend to do this when markets are rising and using leverage seems like an effortless way turbocharge their gains. But eventually the market turns down, leaving stock portfolios insufficient to cover related margin debt and generating “margin calls” in which brokers demand more money and/or start liquidating customer portfolios. This sends the market down sharply and indiscriminately, as fairly-valued babies are dumped along with overvalued bathwater. The result: a quick, brutal bear market.

Margin debt hit record highs several years ago and has just kept on going. From yesterday’s Wall Street Journal: