What is thesis drift?
Thesis drift is when the original reasons you bought a stock quietly stop being true and you keep holding anyway. It has two faces: the erosion of the facts behind the thesis, and the rationalizing that hides that erosion from you.
Also called: thesis creep, thesis erosion
The two faces of thesis drift
The first face is factual. The specific reasons you bought (the margin expansion, the contract, the moat) weaken one by one as new filings and news arrive. Nothing dramatic happens on any single day, so the position keeps looking fine.
The second face is behavioral, and it is the one most investors miss. As the original reasons fade, you unconsciously swap in new ones to justify holding. The stock you bought for growth becomes a value play; the value play becomes a turnaround; the turnaround becomes "it has to bounce eventually." This is the sense in which value investors like the writers at MicroCapClub and MD&A use the term: not just that the facts changed, but that you let the thesis quietly rewrite itself.
Both faces describe the same failure: you end up owning a stock for reasons you never actually decided to own it for.
Why it is dangerous
Thesis drift is slow, so it never triggers an alarm. A price stop-loss fires on a number. Thesis drift has no number. By the time the chart confirms the problem, the reasons were already gone for quarters.
It is also self-concealing. Because you rationalize as you go, the drift feels like conviction. The investor who is drifting and the investor who is right both feel certain.
How to catch it
Write the reasons down before you buy, and write the single fact that would prove each one wrong (a breaks-if condition). Then check the original reasons against new evidence on a schedule, not against the price.
The discipline is to test the thesis you actually wrote, not the one you have since talked yourself into. That is the entire job of thesis monitoring.
Common questions
What is the difference between thesis drift and a stop-loss?
A stop-loss exits on price. Thesis drift is about the reasons behind the position eroding, which has no price level. You can be down with the thesis intact, or flat with the thesis already broken. A conviction stop, not a price stop, is what addresses thesis drift.
Is thesis drift the same as thesis creep?
They are used interchangeably. Both describe a thesis quietly changing into something you never decided on, either because the facts eroded or because you rationalized new reasons to hold.
How Helm detects thesis drift: open it