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I built a system that pulls every SEC Form 4 open-market purchase, scores it, and measures what the stock actually did afterward versus the market. 4,223 buys, 2016 to 2026, US-listed, investment funds excluded, sub-$1 prices filtered. One methodology note that turns out to matter: I score every buy per company (one vote each), not per filing. Three popular beliefs mostly fell apart. One held up.
**Myth 1: "Follow the CEO."** The CEO buying is supposed to be the strongest signal. In my data it was one of the weaker ones. Over a 3-month hold, CEO-led buys won just 41% (median -2.4%), versus 55% (median +1.2%) for non-CEO insiders, and that held across score thresholds and both halves of the sample. The "CEOs earn huge returns" claim? In my set that whole average traced back to one stock (AP, +236% on a single filing). Strip it and the CEO average turns negative.
**Myth 2: "Cluster buys, 3+ insiders at once, are a much stronger signal."** It looked strong on the surface. Then I bootstrapped it by company instead of by filing, and the "3+ insiders" premium was not statistically significant (the 90% confidence interval straddled zero, P around 0.71). The ranking even flipped by horizon: 3+ was the worst bucket at a 1-week hold. And the whole cluster "average" was propped up by about three stocks (AP, NGL, CVNA); remove them and it goes negative. A weak, unstable hint dressed up as a flashing green light.
**Myth 3: "Insiders know things, so buying alongside them is a reliable win."** The big one. It's not reliable, it's a lottery. Take micro-caps (under $300M), the best bucket: the average buy beat the market by +7.6% over 3 months, but the median trade was -1.5% and only 47% were winners (62 companies). The gains lived almost entirely in a handful of multibaggers. That flips how you'd use it. Don't back up the truck on one "high-conviction" name. Spread across many, size small, and let winners run, because you can't tell which coin-flip becomes the 5-bagger.
**What actually held up:** the edge lived in small and micro-caps (the neglected corner big money can't easily trade), and it rewarded a "buy many, let winners run" structure over concentrated bets. The one genuinely bright spot was a 1-week hold of high-scoring micro-caps, which won roughly 62% to 73% with a positive median. But that's only about 30 companies, so I treat it as a lead, not a law.
**Caveats, because they matter:** one dataset, one decade. The micro-cap slice is about 62 companies (the best sub-slices around 30), so dozens, not thousands. These are strong hints, not laws. I'm now testing the whole thing forward with a live paper portfolio to see if it survives out-of-sample.