Margin debt is money that investors borrow in order to invest in stocks. When investors are optimistic about the short term future of the stock market, they will borrow money and invest it in stocks. When they are pessimistic, they will repay those debts. As of October 31, 2023 (the latest data available), total US margin debt was $635 billion, which represents a decrease of $35 billion year-over-year.

Since margin tends to increase as the size of the stock market grows, it's more useful to look at the margin debt in terms of the total stock market value. Using that view, over the last 12 months total margin debt decreased by 0.08% of the value of the entire US stock market. That change is 0.36 standard deviations below the historic average, indicating that the stock market sentiment is currently Neutral about short term returns.

This is summarized in the chart below. For a step-by-step analysis of the model and how the chart was created, continue reading.

Theory & Data

Margin debt is the money that investors borrow in order to invest in stocks. This debt is usually collateralized by the stock investments themselves. When total margin debt increases, the overall level of risk in the stock market increases as well. Then, if the stock market falls, the value of the collateral behind margin loans falls, and so investors need to pay back the loan (a "margin call") by selling stocks, which itself drives the overall market lower. Because of this positive feedback loop, the higher overall market margin debt increases, the further and faster one could expect the market to fall during the next downturn.

Even without the margin-call mechanism that can accelerate market declines, looking at overall margin levels is useful since this is a proxy indicator for market momentum. High margin shows that investors are extremely bullish - so much so that they are willing to borrow money in order to invest in additional stocks. Whereas falling margin levels suggests the opposite, that in aggregate investors have little faith that the market will continue to rise, and so are not willing to borrow money (and pay interest) in order to invest further.

All data is cited below, and comes from FINRA and NYSE. Margin debt levels are published monthly (after some delay), with the most recent available data being from October 31, 2023.

Total Margin Debt

Total margin in US accounts since 1970 is shown below. All figures throughout this article are inflation-adjusted to reflect present day dollars. As is shown, margin was seldom used prior to the mid-90's, when it began ramping exponentially ahead of the internet bubble. After the crash margin remained in high use, and reached an even higher peak ahead of the Great Financial Crisis in 2008. This pattern repeated after the GFC crash, and margin has now climbed to the highest levels ever recorded.

Of course, the total size of the US stock market has grown dramatically during this same timeframe, even adjusted for inflation. It makes sense that the amount of cash being borrowed to invest in stocks would rise as the total size of the market increased. The below chart looks at total margin debt as a percentage of the total value of the US stock market (estimated via the Wilshire 5000 index).

Rather than look strictly at the level of overall margin, we want to get a better sense of how quickly margin is changing over time. Are investors gaining confidence, and borrowing more cash in order to increase long positions? To do this we look at the rolling year-over-year change in total margin debt. That is, how much has margin debt changed in the prior twelve months?

From the chart above it looks like margin debt has gone out of control in the last few decades. Recall from above that as a percent of the value of the total stock market, margin debt has risen significantly since 1970, but actually not very much since 2000. Once again, let's take a look at the above chart (year-over-year change in margin debt) but proportioned to the size of the stock market.

Now we can see that margin debt swings tend to stay pretty reliably within a small single-digit percent of the value of the stock market overall. A huge uptick in margin debt would be when, over twelve months, the total amount of margin debt outstanding goes up by ~2% of the value of the entire stock market. The fastest debt recedes out of the market is roughly the same.

Our main interest is in what margin levels might say about overall market valuation. After a giant run up in margin debt, we should expect the market to underperform, since the selling pressure during a deleveraging will accelerate a market correction/crash. Below is the same chart as above, only now with highlights showing periods of time where 1-year stock market returns have been negative. Notice that almost every peak in margin debt is followed by periods of negative market returns as margin debt unwinds.

We generally look at valuation indicators via a ranking system based on the number of standard deviations that values are from the mean. We can create such a model using the available margin data here. This is identical to the chart in Figures 4 and 5, just now with horizontal bands showing the number of standard deviations from the mean.