One of the most reliable ways to gauge how expensive the stock market has become is the CAPE ratio — the cyclically adjusted price-to-earnings ratio, also known as the Shiller PE. It smooths earnings across a full business cycle to avoid being misled by a single good or bad year.
Recently, the CAPE ratio has been rising in a nearly straight line. That matters because it signals that prices are climbing faster than underlying earnings can justify.
Why the CAPE keeps climbing
Even in a period of genuine earnings growth, the CAPE can still go up. That happens when stock prices rise faster than earnings. When investors push prices higher on optimism about the future — often driven by a handful of high-profile stocks — valuations stretch even as the fundamentals improve.
That is worth distinguishing from a straightforward rally. A market that rises because earnings grew is healthier than one that rises simply because investors are willing to pay more for the same earnings.
A momentum-driven market
Part of the recent surge has been powered by a narrow group of stocks, particularly in artificial intelligence. When a few large names drive the index, overall market breath — the number of stocks participating in the rally — can stay weak even as the headline number climbs.
Weak breadth is a warning sign. It means the rally is fragile, because it depends on a small set of winners rather than broad economic strength.
Fundamentals versus sentiment
Analysts and investors can disagree on how to read the same data. Some point to strong earnings momentum as justification for higher prices. Others warn that not all capital spending is productive, and that a wave of AI-related investment may be building capacity the economy does not fully need.
Both views can be partially right. Earnings are genuinely improving. But if a large share of that improvement is being funded by speculative capital spending, the quality of the earnings is lower than it appears.
What to do about an expensive market
A high CAPE ratio is not a timing signal — markets can stay expensive for a long time. But it is a reason to temper expectations and keep a closer eye on risk. If you are heavily concentrated in the momentum names that drove the rally, it may be worth rebalancing toward a more diversified position.
Valuation tells you what to expect over the long term, not what happens next week. Respect it, but do not try to trade around it.