Insider Buying vs Selling: Which Signal Matters

Insider transaction data is one of the few genuinely informative public disclosures available to retail investors. It is also, I think, the one most consistently misread, and the reason is simple enough: people treat buying and selling as though the two carried equal weight, when the asymmetry between them is the entire point and the thing that makes the data useful at all.

The asymmetry

Consider the reasons an executive sells. A tax bill on vesting shares. Diversifying a net worth that is 80% one stock. A house purchase, a divorce settlement, a plan arranged eighteen months earlier and executed on schedule. None of these say anything whatever about the company’s prospects, and the majority of insider selling is mechanical in exactly that way.

Now consider the reasons an executive buys. There is essentially one. The purchase spends after-tax money, increases an exposure that is already concentrated, and does so in the single company about which the buyer knows more than anybody reading the filing, which leaves no tax story and no diversification story available as an explanation.

Reading a Form 4

Insiders in the US must file a Form 4 within two business days of a transaction, and the filing is free and public on the SEC’s EDGAR system.

Four things are worth reading, roughly in order of importance.

Transaction code first. P is an open-market purchase and it is the one that matters, S is a sale, M is an option exercise and A is an award or grant. Option exercises and grants are compensation. Summary sites miscount them as insider buying with some regularity.

Then whether a 10b5-1 plan was involved. A pre-scheduled sale was decided months earlier, under circumstances that no longer obtain, and it carries almost no information. The filing normally indicates this, and checking costs seconds.

Then size, always relative to existing holdings. A director buying $50,000 while already holding $40 million is noise. An officer increasing their position by 40% is a statement.

Then who. A CFO signal generally outweighs a CEO’s, since chief financial officers see the numbers first and tend to be more conservative about the optics of buying their own stock, while independent directors carry less weight than operating executives do.

The patterns worth tracking

Cluster buying. Several insiders buying independently of one another within a few weeks. Any one person can be wrong, over-optimistic, or simply acting on personal circumstances the filing does not reveal, whereas a CFO, two directors and the CEO all buying inside a fortnight becomes rather difficult to explain away, and this is the strongest form the signal takes.

Buying into weakness. Purchases made while the stock is falling. Insiders are disagreeing with the market in public, with their own money, at a moment when waiting would have been a good deal easier.

A first purchase in years. An executive who has only ever sold, suddenly buying, represents a change in behaviour, and changes in behaviour tend to be more informative than levels.

Unusual selling. One selling pattern does carry real weight: multiple insiders disposing of large proportions of their holdings, outside scheduled plans, within a compressed window. That is a different thing in kind from routine vesting sales.

What it cannot tell you

Timing. The information concerns value, and the point at which the market comes round to agreeing is left open.

Magnitude. A purchase says “I think this is undervalued.” It does not say by how much.

The whole picture. Insiders are subject to the same biases as everyone else, with a structural one added on top, since they are institutionally optimistic about the company they run, and a good many executives have bought aggressively all the way down to zero.

Using it in practice

Insider data works best as a screening input. As a trade signal it is poor.

1. Filter for open-market cluster buys: code P, multiple insiders, non-trivial size.
2. Run the fundamentals. Does the cash flow and margin picture support the insiders’ apparent view, or contradict it?
3. Check for a value trap. Insiders buying a structurally declining business amount to a falling knife with an endorsement attached.
4. Wait for price confirmation. Since insiders are early, entering once price stops falling costs some upside and removes much of the drawdown.
5. Size normally. The signal does not justify an oversized position, so apply the same position sizing you would to any other idea.

What the data realistically does is narrow a universe of thousands of companies down to a shortlist worth examining properly, which is useful, and a good deal less than the way it is usually sold.