The S&P 500 (Index: SPX) has reverted back to its mean.
We can say that because the S&P 500 has experienced a relatively stable period of order since the end of the fourth quarter of 2023. We know that's the case because the variation of stock prices with respect to the mean trend curve established from the relationship between stock prices and their trailing year dividends per share can be generally described by a normal distribution.
That in itself is remarkable because stock prices are very much not normal, even when they behave in an orderly manner. When you map their variation onto a chart with zones that align with significant reference points for a normal bell curve from statistics, you'll find both too many points within one standard deviation of the mean and too many points outside the zones where they would be expected to be found 99.8% of the time if that variation was really normally distributed.
But that doesn't mean we can't use the tools built for doing statistical analysis to track an index like the S&P 500 when such a period of order exists in the stock market. The following chart deploys those tools and finds that as of the close of trading on 11 March 2026, the level of the S&P 500 is just a short distance from its central mean trendline. Which is to say the level of the S&P 500 has finally returned to its established mean after having run above it since early September 2025.
This is an almost textbook example of what "reverting to the mean" really means where stock prices are concerned.
But in case you're wondering what it means when stock prices move outside the outer limits described by this kind of analysis, where order really does break down (as opposed to simply being the result of statistical outliers in a continuing trend like what happened back in April 2025), the ultimate textbook example involves the ultimate sell signal.