U.S. equities have become so expensive by one standard market measure that valuations now exceed levels recorded ahead of the 1929 collapse.
Supporters of the rally argue that artificial intelligence will deliver sufficient earnings growth to sustain the rally, but past market patterns suggest today's enthusiasm may come at a high long-term cost.
What's happening?
According to The Telegraph, the S&P 500 sits near 41 on Robert Shiller's CAPE measure, which compares prices with earnings over a longer cycle.
On that basis, the index is priced above the peaks seen in 1901 and 1929 and is at its loftiest level since the dot-com bubble around 2000.
The same report found U.S. shares are valued at more than twice the level of similar stock markets in the U.K. and Europe, a spread wider than any previously recorded.
Profit estimates, meanwhile, keep moving up. Analysts now expect S&P 500 companies to post record earnings of about $323 per share in 2026 and roughly $376 per share in 2027, with much of the excitement centered on technology companies.
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Much of that confidence rests on AI. Investors are effectively assuming the technology will drive a major jump in productivity and corporate profits, making current valuations look reasonable.
Why does it matter?
Retirement accounts, pensions, and index funds are heavily tied to large U.S. companies, which means that when valuations become stretched, the risk is not just a sudden sell-off but also a prolonged stretch of weaker returns.
CAPE has generally been more informative about the next 10 years than the next 10 days. Historically, paying unusually high prices for earnings has been followed by lower long-run returns, so while that says little about whether stocks fall soon, it does raise the odds that the coming decade will be less rewarding than the last.
AI may yet lift profits by helping businesses work more efficiently and improve products and services. Still, the growth now implied by market expectations is far above the pace of earnings gains seen over long periods, leaving stocks vulnerable if those profit gains arrive slowly or not at all.
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What can I do?
Rather than leaving a portfolio heavily concentrated in an overheated part of the market, investors can spread risk by adding international funds, bonds, or other asset classes.
A steady rebalancing plan may be more practical than trying to identify the exact peak. Shiller's CAPE is not meant to forecast next week's or next month's moves, but it has proven valuable for framing likely returns over a 10-year stretch.
On the energy side, paying attention to local utility plans, supporting home-efficiency upgrades, and backing cleaner, more resilient power systems can reduce both pollution and grid stress.
Major technological advances do not erase the fundamental relationship between price and return.
Or, as the analysis puts it, "The higher Cape goes, the lower the CAGR over the next decade."
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