Paper Trail #6: Post-Earnings-Announcement Drift (Bernard & Thomas, 1989)
Victor Bernard and Jacob Thomas published a paper in 1989 that took a decades-old empirical puzzle — the fact that stock prices keep drifting in the direction of an earnings surprise for weeks after the announcement — and demonstrated that no risk-based story could reconcile it with efficient-markets theory. The paper is Post-Earnings-Announcement Drift: Delayed Price Response or Risk Premium?, in the Journal of Accounting Research, Volume 27 Supplement, pages 1-36. Their long/short portfolio — buy the highest-SUE decile, sell the lowest — earned a positive spread in 41 of the 48 quarters between 1974 and 1985.That’s roughly a 6-out-of-7 quarterly win-rate for a signal that every efficient-markets model of the era said should not exist.
Verification note.The primary paper sits behind JSTOR’s paywall, and the follow-up paper (Bernard & Thomas 1990, Journal of Accounting and Economics13: 305-340) is similarly gated. Every numerical or attributed claim in this post traces to one of three open-access secondary sources: Wikipedia’s PEAD article, Ian Gow’s textbook chapter, or the Katz replication paper. I could not read the 1989 PDF itself today. Where a claim is quoted verbatim, the secondary source is named in-body.
The observation Bernard and Thomas started with
Ball and Brown (1968) — the same pair Fama would later cite in his 1970 efficient-markets paper (see Paper Trail #4) — had already noted that stock prices continued to drift up in the weeks following a positive earnings surprise, and down after a negative one. Fama-1970’s reading was that the drift was real but modest, consistent with the semi-strong form of market efficiency once you accounted for risk. B&T 1989 revisited that reading with 15 more years of data and a more rigorous methodology.
Their expected-earnings model was a seasonal random walk: last year’s same-quarter earnings, adjusted for a trend term. That’s a deliberately simple forecast — no analyst estimates, no exponential smoothing — designed to capture what a naïve investor would have expected the quarter to produce. The surprise at each announcement is then the difference between the actual reported earnings and that naïve forecast, standardised by the historical volatility of prior surprises. That standardised measure is the SUE.
The decile-portfolio construction
Ranking firms into ten portfolios by their current-quarter SUE creates an obvious look-ahead problem: if you build the deciles using the CURRENT quarter’s SUE distribution, you already know each firm’s relative rank, which no live investor could have known before the announcement. B&T handled this by cutting the deciles on the priorquarter’s SUE distribution instead — a clean out-of-sample bucketing that mimics the information set an investor had at the time of the earnings release.
(Per Ian Gow’s textbook summary of the methodology, B&T also required each firm to have at least 10 quarters of prior earnings data to compute the expected-earnings forecast, and used up to 24 quarters; where fewer than 16 quarters were available, they defaulted to a simpler forecasting model. Small methodological details like this are the kind of thing that makes a paper reproducible; PEAD’s replication history is unusually clean partly because B&T were careful.)
The headline finding
Once the portfolios were built, the top-SUE decile outperformed the bottom-SUE decile over the ~60 trading days after the announcement in 41 of the 48 quarters between 1974 and 1985. That’s a roughly 85%quarterly win-rate — the drift is not a cherry-picked subsample, it’s the base rate across a full 12-year window (per Wikipedia’s summary; the numeric figure is the widely-quoted headline result from B&T 1989).
More striking: the top-minus-bottom spread was also positive in 11 of the 16 quartersin which the NYSE index itself was negative. Which means PEAD wasn’t just “the good-news portfolio drifts up in a rising market.” The spread was there in bear quarters too — the good-news decile fell less than the bad-news decile, so the long/short still won.
What Bernard and Thomas ruled out
The paper’s central intellectual move was to test whether the drift could be explained by risk mismeasurement. Ball (1978) had already proposed that PEAD might be a compensation for the additional risk carried by high-surprise stocks — if you’re long the earnings-surprise portfolio, maybe you’re long some latent risk factor that CAPM doesn’t capture. B&T tested this systematically: controlling for firm size (already known to be correlated with SUE), for beta, for delisting bias, for the various methodological adjustments that had accumulated in the drift literature over the previous decade.
None of it explained the drift away. Their conclusion, as summarised in the Katz replication paper (which quotes them directly): “investors...fail to recognize fully the implications of current earnings for future earnings.” Which is behavioural-finance-speak for “the market underreacts to earnings news.” And underreaction cannot coexist with strict semi-strong-form efficiency, which is the null hypothesis Fama-1970 had defended. PEAD is one of the earliest formally-documented violations of that null on institutional-quality data with careful methodology.
How PEAD held up
Later research showed PEAD didn’t just persist — it wasthe least-controversial anomaly in behavioural finance for the next thirty years. Fama (1998), whose whole research programme was to defend efficient markets, called PEAD “the granddaddy of underreaction events” and conceded it as one of the few genuine anomalies that survived every attempt to explain it away as a mismeasured-risk artefact.
The drift’s magnitude has shrunk over time — arbitrageurs piled into the trade in the late 1990s and 2000s, and earnings persistence itself has declined (see the Columbia working paper “Why Has PEAD Declined Over Time?” for the current state of that debate). Katz’s replication paper cites a modern range of ~8.76% annualised(Sadka 2006) as a lower bound for the trading strategy’s post-cost return, with earlier literature reporting higher figures. The drift is smaller than it was in the 1974-1985 sample, but it’s still there.
What it means for the FX empirical work Vantage does
PEAD and the News Impact Explorer are asking the same question in different asset classes. Both are: “When a scheduled data release surprises the market, does the price move fully in the first candle, or does it take longer?” Both find: it takes longer. Both then ask “how much longer, and does the miss/beat asymmetry survive controls?” Both find: yes, and yes.
There are differences — PEAD lives in quarterly-cadence equity-market data with position holds measured in weeks; Vantage’s FX work lives in per-release intra-day data with holds measured in minutes. But the empirical template is the same, and PEAD’s three-decade robustness is the strongest existing evidence that the surprise-driven-drift family of findings is genuinely tradeable structure, not a data-mining artefact. That framing — my extrapolation, framed as such — is what B&T 1989 buys us in the equity-market parallel.
What this doesn’t say
PEAD is not a live retail-tradeable edge in 2026. The strategy’s expected return has decayed as institutional arbitrage crowded in and as retail commissions, while lower than in 1989, still eat into a signal that requires holding through the whole ~60-day drift window with quarterly turnover. Discussion of PEAD as a “you should trade this” strategy conflates the 1989 headline result with current implementation feasibility.
Verification caveat.As noted above, specific numerical claims in this post are cited from secondary sources (Wikipedia, Ian Gow’s textbook, the Katz replication paper) rather than the 1989 PDF, which I could not access today. If the primary paper says something different from what the secondaries report, the primary paper is authoritative and this post should be corrected. The “41 of 48 quarters” and “seasonal random walk” claims specifically are widely reproduced across multiple secondary sources with consistent citation, so I’m confident in those; the surrounding methodology and interpretive claims all trace to Ian Gow’s chapter or Katz’s paper explicitly.