HML Devil and Stale Book Values
The classic value factor pairs a book value that's up to 18 months old with a stock price updated every second, and that mismatch alone changes how the factor behaves — a version called 'HML Devil' fixes the timing and gets a different factor.
Prerequisites: The Value Factor
The original value factor, HML ("High Minus Low"), sorts stocks by book-to-market ratio: book value of equity divided by market price. But the classic Fama-French construction takes the book value from a company's fiscal year-end 15 months ago and divides it by the market price today. A stock's price updates every trading day; its book value on the numerator can be over a year stale by the time the ratio is used. That mismatch — a moving number divided into a frozen one — is what practitioners call the "devil" in HML's details.
Classic HML mixes a book value that's up to 18 months old with a live market price. Updating book value more frequently and using a more current price produces a different, more responsive value factor with materially different risk and return behavior — same idea, different plumbing.
Where the staleness comes from
Fama and French built HML in the 1990s, when timely accounting data was hard to get reliably across the whole market, so they used a conservative rule: to be sure every company's annual report was actually public, wait until June of the following year to use last fiscal year's book value, and hold that number fixed for a full twelve months until the next June. A company with a December fiscal year-end therefore has its book value fixed from June (6 months after year-end) to the following June — at the start of that window the book number is already 6 months old, and by the end it's 18 months old.
Worked example
A retailer reports book value of $10/share at its December 2024 year-end. Under classic HML timing, that $10 figure is used from June 2025 through May 2026 — even though a February 2026 buyback or a large writedown might have changed true book value substantially by then. Suppose the stock trades at $15 in June 2025 (book-to-market of 0.67) and falls to $9 by May 2026 after a rough year. Classic HML now shows book-to-market of — a jump into "cheap" territory driven entirely by the falling price, one year after the book figure was even measured. A version updating book value quarterly, using each quarter's actual reported figure, would instead be reacting to real accounting changes rather than a stale anchor.
What this means in practice
Two "value factors" with the same name can behave quite differently depending on how current the book-value input is. The updated version (nicknamed "HML Devil" by AQR) tends to react faster to fundamentals and shows less of a lag-driven correlation with momentum, which matters for anyone combining value and momentum signals — the classic version's staleness makes it look more like a slow-moving reversal signal than a pure valuation measure.
"HML" is not one factor — data vendors and researchers implement the timing rule differently, and results that look like a value-factor discrepancy across studies are often just a book-value-timing discrepancy in disguise.
Related concepts
Practice in interviews
Further reading
- Asness, Frazzini, 'The Devil in HML's Details' (Journal of Portfolio Management)
- Fama, French, 'Common Risk Factors in the Returns on Stocks and Bonds' (Journal of Financial Economics)