What Is the CAPE Ratio?
The cyclically adjusted price-to-earnings ratio compares the current price of a stock index, most commonly the S&P 500, to the average of its real, inflation-adjusted earnings over the trailing ten years. Economist Robert Shiller popularized the measure with John Campbell in the late 1980s, building on Benjamin Graham's advice to average earnings over multiple years before judging value.
The ten-year averaging is the key design choice. A standard trailing P/E can look deceptively low at an earnings peak and alarmingly high in a recession when profits collapse, exactly backwards as a value signal. By smoothing a full business cycle of earnings, CAPE gives a steadier read on how much investors are paying for a dollar of sustainable profit.
How It Is Calculated and Interpreted
The formula is CAPE = Real Index Price / Average Real Earnings over the prior 10 years, with both price and earnings restated into today's dollars using CPI. For the S&P 500, the long-run historical average sits around 17. The ratio reached roughly 33 before the 1929 crash, peaked near 44 during the dot-com bubble in 2000, and climbed back into the high 30s during 2021, each episode followed by weak subsequent returns.
Empirically, higher starting CAPE levels have been associated with lower average stock returns over the following ten years, which is why long-horizon allocators watch it. It is a poor timing tool, though: CAPE stayed elevated for most of the 2010s while the market kept rising. Critics also note that changes in accounting standards, buyback-heavy payout policies, and structurally lower interest rates can justify higher ratios than the century-long average implies.
Why It Matters
For anyone entering asset management, macro research, or wealth advisory, CAPE is part of the standard vocabulary for market-level valuation debates. Strategists cite it when setting long-term capital market assumptions, endowments use it in asset allocation studies, and clients ask about it whenever headlines declare stocks overvalued. Knowing both the evidence for CAPE and its well-documented limitations lets you engage in those conversations credibly.
The ratio also illustrates a broader analytical principle that applies to single-stock work: normalize the earnings base before trusting a multiple. The same instinct that leads Shiller to average ten years of index earnings leads an equity analyst to value a cyclical company on mid-cycle profits, making CAPE a useful bridge between macro valuation and bottom-up fundamental analysis.
