A first number — usually the price you paid — sticks in the mind and quietly bends every later judgement. Tversky and Kahneman named this anchoring in 1974, and in investing the most damaging anchor is your own purchase price.
Three anchors do the most harm. The purchase price — “I paid $80, so I’ll sell when it gets back to $80” — treats a number that matters only to you as if the market cares about it. The 52-week high — “it was $120 once, so $90 is cheap” — assumes an old peak is the “real” value. And round numbers ($100, $50) feel meaningful only because they are round. None of these tells you anything about whether the business is worth owning today.
Example (illustrative): you buy at $80 and the company’s outlook then genuinely worsens — the reason you bought it no longer holds. The stock is $62. Anchored on $80, you refuse to sell “at a loss” and wait for a recovery that has no reason to come, while fair value keeps drifting lower. The $80 is irrelevant to the decision in front of you; the only question is whether $62 is worth holding on today’s facts.
Replace the backward-looking anchor with a forward-looking one. Before you buy, write down why you own it and the invalidation level — the price or event that would prove that reason wrong. Then judge the position against that, not against what you paid. Levels where lots of other traders have historically acted — genuine support and resistance — can matter because the market reacts to them; your personal entry price does not, because no one else knows or cares about it.
This is why a disciplined tool frames every idea around a forward-looking thesis and an explicit invalidation level rather than around your cost basis: the structure itself refuses to let your purchase price run the decision.
This lesson is general investor education, not personalized investment advice. The $80/$62 prices are illustrative arithmetic to show the anchoring trap, not a forecast or a recommendation about any real stock.
It’s the tendency, shown by Tversky and Kahneman in 1974, for a first number you encounter to become a reference point that skews later decisions. The classic investing example is fixating on the price you paid and refusing to sell until it “gets back to even,” regardless of whether the investment is still sound.
Because the market has no idea what you paid and doesn’t price the stock around it. What a position is worth today depends on today’s facts about the business, not on your personal cost basis. Selling “at a loss” or “at a gain” relative to your entry is an anchor, not a reason.
They’re different: support and resistance are prices where many market participants have historically acted, so the market as a whole can react to them. Your purchase price is known only to you, so it carries no such collective meaning — that’s why one can be useful context and the other is a personal trap.
Investor education only — not investment advice, and never a promise of profit. Every investment can lose value.