The S&P 500 vs Its Long-Term Trend

The S&P 500 grows in a remarkably straight line — on a log chart. Fit that line to its whole history, and how far today's price sits above or below it tells you how stretched the market is versus its own past.

What is an exponential trend?

Stocks don't grow by a fixed number of points each year — they grow by a percentage. Anything that compounds like that traces a curve on a normal chart but a straight line on a log chart. Fitting a straight line through the S&P 500's log price (an exponential regression) gives its long-run growth path — historically around 7–8% a year before inflation.

Around that line, st-ox draws bands one and two standard deviations wide. The result is a channel the index has swung through for decades: expensive up near the top, cheap down near the bottom.

How to read it

• Above the trend line — the market is pricier than its own long-run path. The further above, the more stretched.
• Near the top band (+2σ) — historically expensive; readings this high have been rare.
• Around the line — roughly in line with its long-run trend.
• Near the bottom band (−2σ) — historically cheap; the kind of level that has marked major lows.

The gauge above reduces all of that to one number — how far above or below trend the S&P 500 is right now — and a verdict based on where that sits in its own history.

Why it matters

Like other valuation gauges, this is about long-run expectations, not next week. When the index has been far above its trend, the following decade has tended to be below-average; far below, above-average. It doesn't tell you to buy or sell today — it tells you whether you're starting from a rich or a cheap base, so you can set realistic expectations and size risk accordingly.

Trend vs the Buffett Indicator and Q ratio

This gauge compares the market to its own price history. The Buffett Indicator and Tobin's Q ratio instead compare it to fundamentals — the size of the economy and the replacement cost of company assets. They answer slightly different questions, which is why st-ox blends all three into one verdict on its Market Valuation page. When price-vs-trend, the Buffett Indicator and Q all point the same way, the signal is stronger.

Caveats

• Nominal, not real. These are raw prices; the fitted growth rate includes inflation.
• The trend drifts. The line is fitted to history and shifts as new data arrives — it's descriptive, not a law of nature.
• Not a timing tool. The market can stay above (or below) trend for years.
• Other markets have shorter histories. st-ox also charts Europe, Germany, France, the UK and Japan, but their trends rest on fewer decades and are less robust.

Frequently asked questions

For information and educational purposes only. Not investment advice, and not a recommendation to buy or sell any security. Data is sourced from Yahoo Finance and may be delayed or revised.