Warren Buffett teaches sizing through a thought experiment, usually attributed to him via his colleague Charlie Munger and their friend the businessman-investor circle: imagine you had a punch card with room for only twenty punches in your entire investing life, one punch per investment decision, twenty for a lifetime, no more. How would you behave? You would become extraordinarily selective. You would pass on the merely-good to save punches for the genuinely great; you would research each candidate exhaustively because a mistake is one-twentieth of your whole allowance; you would size each survivor meaningfully, because with only twenty ideas ever, a 2% position is a waste of a punch. Buffett's claim is that an investor forced to think this way would end up far richer than one who trades constantly, precisely because the scarcity forces quality.
Why is the punch card an argument for concentration?
The twenty-punch idea is really an argument for concentration, and it connects straight to Kelly: if you genuinely have only a handful of high-conviction, capped-downside ideas at any time, the math wants you to bet them meaningfully rather than diluting your best insight across a hundred names where your fortieth-best idea drags down the returns of your first. Buffett has run Berkshire this way, with a large share of the equity portfolio in a few names, and Munger more aggressively still, holding as few as three positions and calling wide diversification a confession that you don't know what you're doing. Their sharpest line is the one to remember: "Diversification is protection against ignorance. It makes very little sense for those who know what they're doing."
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When does concentration work, and when doesn't it?
But teach this evenhandedly, because the punch-card discipline is a loaded weapon:
- It works only if you actually have edge and stay inside your circle of competence. Concentration multiplies the consequences of your judgment in both directions: it makes a good analyst richer and a mediocre one poorer, faster. Buffett can concentrate because he is Buffett and because he pairs it with an obsessive margin of safety and a horror of leverage.
- For the vast majority of investors (including you, until you have a long, honestly-journaled track record proving genuine edge) Buffett's other famous advice applies: most people should own a low-cost index fund and not concentrate at all, because they don't have demonstrable edge, and for them diversification is exactly the right protection against exactly the ignorance he names.
- The resolution is not "concentrate" or "diversify" as a slogan; it is concentrate in proportion to your demonstrated, sized-for edge, and never past the point where a correlated cluster of your best ideas could ruin you (hidden correlation, the anti-ruin overlay). The punch card disciplines quality; the caps discipline survival; you need both, and this whole debate is picked up formally in M9.02.
The takeaway for sizing: let scarcity be your friend. If an idea would not survive the punch-card test, if you would not spend one of your twenty lifetime punches on it, it does not belong in your portfolio at a meaningful size, and a non-meaningful size is usually not worth the slot at all. Concentration is not a target; it is what naturally happens when you demand that every position be genuinely excellent and size it to its edge.