Margin debt, incredibly, is up 100 times in the last 39 years (see the chart above). It was $4 billion back in 1974; it's nearly $400 billion today. That is a big reason why stock prices are in the stratosphere. You might think that there's a lot more money around, thereby justifying the rise. ... Let's normalize this indicator to GDP and see what we have. ... [M]argin debt as a percentage of annual GDP is still 10 times the 1974 level. ... The current ratio is also 3 times what it was at previous major tops in the stock market in the 20th century.