Could This 1 Thing That's Happened Only 3 Times in the Past 67 Years Halt the S&P 500's Gains in 2025? Here's What History Says.

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This year has been a fantastic one for stocks, from the moment the S&P 500 confirmed a bull market back in January to today, with the three major benchmarks heading for double-digit annual gains. The S&P 500 has climbed 27% this year, while the Nasdaq and the Dow Jones Industrial Average have advanced 34% and 15%, respectively.

The reason for such a performance? Investors early in the year anticipated the first series of interest rate cuts in four years, and they bet on a better economy ahead. On top of this, companies began talking more and more about the high-growth area of artificial intelligence (AI) and pouring investment into AI programs -- and investors piled into stocks that could score a win here. These two elements helped drive the positive momentum we've seen in the stock market in 2024.

Still, even in a strong market environment, there's always something that could put the brakes on some of the excitement. And today, this is something that's happened only three times since the S&P 500 launched as a 500-company index back in the late 1950s. Could this particular thing halt the S&P 500's gains in 2025? Let's find out.

An investor in an office looks pensively at a computer.
Image source: Getty Images.

The bull market is going strong

First, it's important to note that the current bull market is going strong, after the S&P 500 reached multiple record highs this year. Stocks across industries have gained, though growth stocks have shown particular strength -- that's because these companies have an easier time expanding when the economy is doing well. And, as mentioned, AI stocks led the bunch, with names like chip giant Nvidia and AI-driven software company Palantir Technologies recording the best performances in the Dow Jones Industrial Average and S&P 500, respectively.

As a result of all of this momentum, valuations have taken off. And this leads us to the one thing that's happened this year and only two other times in the past 67 years. The S&P 500 Shiller CAPE ratio has increased past the level of 35. The Shiller CAPE ratio takes an inflation-adjusted look at stocks' valuations, considering earnings-per-share over a 10-year period.

S&P 500 Shiller CAPE Ratio Chart
S&P 500 Shiller CAPE Ratio data by YCharts

And today, this measure is telling us that S&P 500 stocks are at one of their priciest levels ever. Now let's consider whether this could halt gains in 2025, and we'll start by taking a look back in time. History shows us that every time the Shiller CAPE ratio reached a peak, the S&P 500 then declined. So, when stocks become too expensive, history shows us the index will fall and valuations, too, will return to lower levels.