Form 4 Filing Speed and Insider Signal Decay

When a company officer, director, or large shareholder buys or sells their own company's stock, US law requires them to disclose it publicly on a Form 4. Academics have found for decades that following insider buying — especially clustered buying by multiple executives — predicts modestly positive future returns, on the theory that people who run a company sometimes know things the market doesn't yet.

Before 2002, insiders had up to ten calendar days after the end of the month in which they traded to file their Form 4 — meaning a trade made on the 3rd of the month might not become public until well into the following month, a lag that could stretch past five weeks in the worst case. Section 403 of the Sarbanes-Oxley Act, passed in the wake of the Enron and WorldCom accounting scandals, cut that deadline to two business days after the trade, effective mid-2003.

Why faster disclosure hurt the strategy, not helped it

It might seem like faster disclosure should be good for a signal-following strategy — you learn about the trade sooner. But the value of an insider-following strategy isn't just "know the trade happened," it's "act on information the rest of the market hasn't priced in yet." Under the old rules, by the time a Form 4 was filed, weeks had often passed — plenty of time for the underlying business development the insider was reacting to (an operational improvement, a contract, a coming product) to become visible through other channels too, so a chunk of the drift had already happened by the time a follower could read the filing. That sounds like it should make old filings useless — but the surprising empirical finding was closer to the opposite: much of the return that had accrued to insider trades before Sarbanes-Oxley came from information still slowly leaking into prices during that long filing lag, and once disclosure became near-instant, the market absorbed the news almost immediately, cutting off the post-filing drift that followers had been capturing.

Worked example

Under the old rules, an executive buys $500,000 of stock on the 2nd of March because she believes an upcoming product launch will beat expectations. The Form 4 isn't required until roughly the 10th of April — five and a half weeks later. In that window, hints of the product's success may already reach analysts, suppliers, or trade press, so some of the eventual price move happens before the filing is even public, but a portion still remains for a fast follower reading the filing the day it posts. Under the post-2003 rules, the same trade must be filed within two business days — by March 4th. The market now sees the insider's conviction almost in real time, alongside whatever the executive knew, sharply compressing the time during which a follower could act on a materially stale-to-the-market signal.

What this means in practice

Studies comparing insider-trading-based strategies before and after the rule change found the abnormal returns to following insider buys shrank measurably post-2002, consistent with faster disclosure closing the informational gap. It's a clean natural experiment for a broader idea: a signal's profitability often depends less on whether the underlying information is valuable, and more on how large a timing gap exists between the informed party acting and the public finding out.