Diversification
The practice of spreading investments across many securities to reduce risk, which Buffett argues is only necessary when you don't know what you're doing.
Concept Analysis
Definition & Origins
Diversification is the practice of spreading investments across many securities and asset classes to reduce the impact of any single position's failure. Modern portfolio theory holds that diversification is the only 'free lunch' in investing — it reduces volatility without reducing expected returns. Buffett's view is nuanced: diversification through index funds is rational for investors who lack conviction in specific ideas, but broad diversification actively dilutes the returns of investors who genuinely understand a small number of exceptional businesses.
Core Ideas
Diversification protects against ignorance. Buffett's precise quote: 'Diversification is protection against ignorance. It makes little sense if you know what you are doing.' If you cannot reliably assess which businesses will outperform over a 10-year horizon, owning all of them prevents catastrophic individual mistakes. For the vast majority of investors — even sophisticated professional ones — low-cost index funds delivering market returns are the optimal choice.
Over-diversification dilutes the returns from excellence. If your analysis correctly identifies five businesses with exceptional 15-year prospects, allocating equally to fifty positions dilutes those five excellent ideas with forty-five mediocre ones. The mathematics are inexorable: your best ideas generate the highest expected returns; diluting them with your worst ideas reduces the total portfolio return toward mediocrity.
Berkshire's apparent vs. actual diversification. Berkshire owns 40-50 publicly traded stocks plus 60+ wholly owned businesses — appearing very diversified. But five positions (Apple, Coca-Cola, American Express, Bank of America, Chevron) consistently represent 70%+ of the equity portfolio. The breadth is in businesses where genuine conviction exists, not in generic sector coverage.
Practical Application
The 1973-74 bear market — when Buffett resumed investing after dissolving his partnerships — is the clearest practical example. While other investors diversified broadly across the declining market, Buffett concentrated in three businesses he understood profoundly: Washington Post, GEICO, and Wesco Financial. Concentration in high-conviction ideas during periods of maximum fear — not diversification across low-conviction ideas — produced the exceptional returns of that period.
Common Misconceptions
Misconception 1: Berkshire is highly diversified. Berkshire's equity portfolio concentration in five positions makes it more concentrated, not less, than a typical institutional portfolio. The 'diversification' is in wholly owned businesses, but those businesses' combined fate is substantially correlated with U.S. economic conditions.
Misconception 2: More holdings always reduce risk. Beyond 15-20 carefully selected positions, additional diversification reduces volatility marginally while increasingly diluting returns for investors with genuine analytical edge. Risk reduction ceases to justify further dilution of concentrated high-conviction positions.
Buffett's Own Words
Billy Rose described the problem of over- diversification: “If you have a harem of forty women, you never get to know any of them very well.”) Over time our policy of concentration should produce superior results, though these will be tempered by our large size. When this policy produces a really bad year, as it must, at least you will know that our money was committed on the same basis as yours. We made the major part of our WPPSS investment at different prices and un
*Further diversification for Berkshire followed, and gradually the textile operation’s depressing effect on our overall return diminished as the business became a progressively smaller portion of the corporation. We remained in the business for reasons that I stated in the 1978 annual report (and summarized at other times also): “(1) our textile businesses are very important employers in their communities, (2) management has been straightforward in reporting on problems and *
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Thought Evolution
Related Concepts
Case Companies
40% of the partnership in a single position: concentration at maximum conviction
Three concentrated positions during the bear market; each subsequently delivered exceptional returns
35-40% of Berkshire's equity portfolio: the most concentrated Berkshire has ever been in a single publicly traded equity