| Question | Total thinking (beginner) | Marginal thinking (disciplined) |
|---|---|---|
| Should the company expand? | We had record profits last year | Does the NEXT factory return more than the cost of capital? |
| Should I keep this stock? | I'm down 50% — I have to wait to get back to even | Does $X here today beat $X in the next-best alternative? |
| Should I hire another salesperson? | The team is producing well | Will the NEXT hire's revenue exceed their fully loaded cost? |
| Should I save more or pay down debt? | Both feel responsible | Which dollar earns more at the margin — the saved or the un-borrowed one? |
The disciplined decision rule from microeconomics is short: deploy resources to any activity where MARGINAL BENEFIT exceeds MARGINAL COST, and stop the moment they cross. Past costs (sunk costs) and historical averages never enter the equation. This is why a software company will rationally sell a license to one more customer at near-zero price (marginal cost of distribution is near zero) — and why an airline will sell the last seat for $50 even though the average seat cost $200 to fly.
The sunk cost fallacy is the single most expensive bias in investing. Holding a losing stock 'until it comes back' is sunk-cost thinking. Continuing a failing business line 'because we've already invested so much' is sunk-cost thinking. Berkshire Hathaway closed its textile business and Apple killed the Mac clone program precisely because the leaders refused to let past spending dictate future allocation. The discipline is hard, but the math is unambiguous: dollars already spent should never influence the next dollar.
Think in margins, not totals. The next dollar is the only dollar that matters for the decision in front of you. Sunk costs are gone — let them be gone. Marginal benefit greater than marginal cost is the universal rule, applied identically to companies investing in factories and to individuals deciding whether to add to a position.
Sit with the ideas.
You bought a stock at $80. It has since fallen to $40 and the original investment thesis is broken. A coworker tells you to 'wait until it comes back to $80 before selling so you don't lose money.' What is the disciplined response from marginal analysis?