Guide

Shiller CAPE ratio explained

Harbor Capital approved a 2021–2025 equity glide path for a $180M defined-benefit plan that assumed a 17x forward P/E was “normal” and projected 7.5% nominal equity returns. The Shiller CAPE (cyclically adjusted price-to-earnings ratio) had already crossed 35 — a level that, in U.S. history since 1881, delivered median real 10-year returns below 4%. When the 2022 drawdown hit, trustees asked why the plan had not flagged valuation risk. Revising return forecasts with CAPE bands cut planned equity contributions by eight percentage points over the glide period and added explicit rebalancing triggers tied to valuation extremes. The Shiller CAPE (also called PE10 or CAPE) divides an index price by the inflation-adjusted average of the past ten years of reported earnings. Nobel laureate Robert Shiller and John Campbell popularized it as a long-horizon valuation gauge that smooths the earnings cycle better than a single-year P/E. CAPE does not time crashes month by month, but it has strong statistical links to subsequent 10- to 20-year real returns — making it a useful input for asset allocation, glide-path design, and expectations around the equity risk premium. This guide covers the formula, comparison to trailing and forward P/E, historical ranges, international CAPE, limitations, a Harbor Capital worked example, a metric decision table, common pitfalls, and an investor checklist.

What the Shiller CAPE measures

The CAPE answers: how expensive is the market relative to a full business cycle of earnings? Standard trailing P/E uses only the last 12 months of earnings. That is distorted when earnings spike in recoveries or collapse in recessions. CAPE averages ten years of real (inflation-adjusted) earnings per share, then divides current price by that average.

CAPE = Price / (Average of last 10 years’ real EPS)

Real EPS adjustment uses the Consumer Price Index (CPI) so past earnings are comparable in today’s dollars. A CAPE of 25 means investors pay $25 for each $1 of normalized, inflation-adjusted earnings power. Higher CAPE implies richer valuations and, on average, lower subsequent long-term returns. Lower CAPE implies cheaper valuations and higher expected returns — though the relationship is noisy over short horizons.

How Shiller CAPE is calculated

Robert Shiller publishes monthly U.S. CAPE data using S&P Composite earnings back to 1871. The steps:

Collect annual reported earnings per share for the index over the prior ten years.
Adjust each year’s EPS for inflation to express in current-dollar terms.
Compute the arithmetic average of those ten real EPS figures.
Divide the current index level by that average real EPS.

Variants exist: some practitioners use geometric averaging, exclude the Great Depression window, or apply sector-specific adjustments. The core idea — smooth earnings over a decade — stays constant. Total-return indices require care: CAPE traditionally uses price indexes and reported earnings, not total-return levels.

CAPE vs trailing P/E vs forward P/E

Metric	Earnings window	Strength	Weakness
Trailing P/E	Last 12 months	Timely, widely available	Distorted at cycle peaks/troughs
Forward P/E	Next 12 months (estimates)	Forward-looking for analysts	Analyst optimism bias; estimates revise late
Shiller CAPE	10-year real average	Cycle-smoothed; long-horizon signal	Lags structural shifts; slow to update

In early 2020, trailing P/E spiked as earnings collapsed during COVID lockdowns — making stocks look artificially expensive on a single-year basis. CAPE moved less because the ten-year average still included strong 2017–2019 earnings. Conversely, after a prolonged earnings boom, CAPE can stay elevated even when forward P/E looks moderate.

Historical CAPE ranges and return predictability

Since 1881, U.S. CAPE has ranged from roughly 5 (1921, 1982) to above 40 (1999 dot-com peak, 2021 post-stimulus). Long-run median is near 16–17. Shiller’s research with Campbell shows CAPE has meaningful explanatory power for subsequent 10- to 20-year real stock returns — not next-quarter returns.

Illustrative historical buckets (U.S., real 10-year forward returns):

CAPE < 12: median real returns often exceed 8% annualized
CAPE 12–20: moderate real returns, roughly 4–7%
CAPE 20–30: lower median real returns, often 2–5%
CAPE > 30: historically weak 10-year real outcomes; outliers exist (1990s)

The 1990s proved that high CAPE can persist for years while stocks still rally — CAPE is a valuation anchor, not a short-term trading signal. Markets can stay expensive longer than disciplined investors stay solvent. The practical use is adjusting return expectations and contribution rates, not calling tops.

CAPE, the equity risk premium, and bonds

Elevated CAPE compresses implied future equity returns, which lowers the forward equity risk premium unless risk-free rates fall in parallel. When CAPE is high and bond yields are also high (2022–2024 regime), stocks face competition from cash and Treasuries for the first time in a decade — a double headwind for passive equity-heavy portfolios.

Some allocators pair CAPE with the earnings yield (E/P, the inverse of P/E) as a crude expected real return proxy: Expected real return ≈ E/P − inflation adjustment. At CAPE 30, earnings yield is ~3.3% before growth. Add long-run real EPS growth (~2%) and you approach ~5% nominal — far below historical equity averages. That math informs rebalancing and spending rules for endowments.

International and sector CAPE

Shiller-style CAPE data exists for many developed markets (Europe, Japan, UK). Emerging-market CAPE is noisier due to accounting differences and shorter histories. International CAPE enables relative-value comparisons: in 2016, U.S. CAPE near 26 vs Europe near 14 suggested cheaper European normalized earnings — though currency, governance, and growth differentials matter beyond the ratio alone.

Sector CAPE (technology vs utilities) helps avoid comparing a high-growth sector’s multiple to a century of whole-market averages. Tech CAPE can look permanently elevated if structural margin expansion persists — but sector CAPE still flags when even growth-adjusted prices stretch.

Limitations and criticisms

Accounting changes. GAAP vs non-GAAP, goodwill write-downs, and buyback-driven EPS growth can shift the ten-year average.
Buybacks vs dividends. CAPE uses per-share earnings; massive buybacks boost EPS without changing underlying economic earnings power the same way dividends would.
Financial sector treatment. Post-2008 bank earnings are volatile; some researchers exclude financials from CAPE calculations.
Low-rate regimes. Extended periods of low real rates justified higher CAPE multiples (2010s); mean reversion timing is uncertain.
Sample size. Only a handful of observations exist above CAPE 30; statistics are thin at extremes.
Not a timing tool. CAPE 35 in 1997 preceded three more years of gains before the 2000 crash.

Treat CAPE as one input among many — alongside forward ERP, credit spreads, and volatility regime — not a single switch for market exposure.

Harbor Capital worked example

Context: $180M defined-benefit pension, 65/35 target equity/fixed income, 15-year de-risking glide to 50/50.

Problem: The 2021 actuarial review used 7.5% nominal equity return based on forward P/E ~17x and historical averages. Shiller CAPE was 38 — second-highest in U.S. history. Real 10-year return regression implied ~3.5% nominal, not 7.5%.

Revision: The investment committee adopted a CAPE-banded return model:

CAPE > 30: plan on 4.5% nominal equity return for glide-path math
CAPE 20–30: 6.0% nominal
CAPE < 20: 7.5% nominal (historical baseline)

At CAPE 38, the glide path slowed equity accumulation from +3% per year to +1% per year, redirecting contributions to liability- matching bonds. They added a rebalancing rule: if CAPE falls below 22 after a bear market drawdown > 25%, accelerate equity purchases by 2% of plan assets. When CAPE normalized to 24 by late 2023, the committee caught up deferred equity contributions without chasing the 2021 peak. Funded status improved versus a static 7.5% assumption that would have masked contribution shortfalls.

Valuation metric decision table

Metric	Horizon	Best for	Main weakness
Shiller CAPE	10–20 years	Strategic allocation, return expectations	Poor short-term timing; slow-moving
Trailing P/E	1–3 years	Quick screens, sector comps	Cycle-distorted at turning points
Forward P/E	1–2 years	Earnings-growth narratives	Estimate bias; revisions lag reality
Earnings yield (E/P)	5–15 years	Stock vs bond comparison	Ignores growth and payout mix
Market cap / GDP	10+ years	Macro cross-check (Buffett indicator)	Ignores international earnings share

When to use CAPE in practice

Retirement and endowment planning: set equity return assumptions that vary with CAPE bands, not a single historical average.
Glide-path design: slow equity accumulation when CAPE > 28; accelerate after drawdowns bring CAPE below long-run median.
International allocation: compare U.S. vs ex-U.S. CAPE for relative value, not as a standalone trade.
Bear-market preparation: high CAPE raises the probability of below-average 10-year returns — tighten position sizing and liquidity buffers; do not abandon equities entirely.
DCF sanity checks: if your stock model needs 12% returns while index CAPE implies 5%, document why your company is an exception.

Common pitfalls

Using CAPE to time monthly entries. Valuation can stay extreme for years; dollar-cost averaging still beats waiting for CAPE 15.
Ignoring interest rates. High CAPE with low rates (2010s) behaved differently than high CAPE with 5% T-bills (2020s).
Comparing CAPE to single-stock P/E. CAPE is an index-level, cycle-smoothed construct; apples-to-oranges comparisons mislead.
Assuming mean reversion on a fixed schedule. CAPE can remain above 25 for a decade; plan for persistence.
Overfitting international CAPE. Accounting standards and index composition differ; adjust expectations.
Abandoning equities at high CAPE. Even expensive markets often deliver positive nominal returns; the issue is magnitude, not sign.
Neglecting earnings quality. Buyback-fueled EPS can depress CAPE artificially if not understood.

Investor checklist

Check current U.S. Shiller CAPE monthly (public data from Yale / multpl.com) and note the long-run median (~16–17).
Map CAPE to your 10-year equity return assumption using bands, not a single historical 10% figure.
Compare CAPE to forward P/E and trailing P/E; large gaps signal cycle position.
Cross-check earnings yield against 10-year Treasury yield for stock-bond opportunity cost.
Review international CAPE if you hold ex-U.S. equity sleeves.
Document CAPE-based assumptions in your investment policy statement.
Set rebalancing triggers for valuation extremes (e.g., add equity after CAPE drops 30% from peak).
Stress-test retirement plans at CAPE > 30 return scenarios (< 5% nominal equity).
Do not reduce equity below your risk tolerance solely because CAPE is high.
Revisit CAPE assumptions annually or when CAPE moves more than five points.

Key takeaways

Shiller CAPE smooths ten years of inflation-adjusted earnings to measure cycle-neutral market valuation.
High CAPE historically correlates with lower subsequent 10- to 20-year real returns — but not reliable short-term timing.
CAPE complements trailing and forward P/E by reducing recession/recovery distortion.
Use CAPE for return expectations and glide paths, not for all-or-nothing market calls.
Pair CAPE with rates, ERP, and volatility context — valuation alone does not dictate regime.