Concentrated Portfolios vs Index Funds: What the Research Really Says, and Why the Index Isn't What It Used to Be

The oldest argument in investing is concentration versus diversification. Warren Buffett has called diversification “protection against ignorance” and concentrates heavily; the index-fund movement says the opposite: own everything, cheaply, forever. For decades this was a matter of philosophy and temperament. Then a finance professor named Hendrik Bessembinder ran the numbers on nearly a century of stock returns and produced a finding stark enough to settle most of the argument. It also explains the strangest feature of the modern market: its extreme concentration in a handful of names. Let’s take the research seriously, then ask the question everyone is really asking: are things different now?

Key takeaways

• Bessembinder’s landmark study found that since 1926, about 4% of US stocks created all of the market’s net wealth above Treasury bills, and most individual stocks underperformed cash.
• That one fact reframes the debate: concentration more often destroys wealth than builds it, because the winners are a tiny, hard-to-predict minority you are likely to miss.
• It also explains the Magnificent Seven: roughly a third of the S&P 500 by 2024, up from about 12% in 2015, and the source of most of the index’s recent returns.
• So “just buy the index” now means buying a concentrated bet on a few mega-caps. A few stocks driving everything is an old story; how concentrated the index you hold has become is not.

The finding that reframes everything

In “Do Stocks Outperform Treasury Bills?” (2018), Hendrik Bessembinder examined the lifetime returns of nearly 26,000 US common stocks going back to 1926. The results are counterintuitive to the point of being disorienting. Four of every seven had lifetime buy-and-hold returns below those of one-month Treasury bills: most individual stocks lost to cash over their lives. The entire net wealth the US stock market created over T-bills, roughly $35 trillion through 2016, came from just 1,092 companies, about 4.3% of the total. And 86 stocks accounted for half of it.

Sit with what that means. The market’s famous long-run return is not a broad tide lifting most boats. It is the product of a small number of extreme winners whose gains are so large they pull the whole average up, while the typical stock quietly trails a savings account. Wealth creation in the stock market is not democratic. It is spectacularly concentrated in a few names.

Why this is the strongest argument for indexing, and against stock picking

Here’s the twist people miss. On its face, Bessembinder’s finding is a case for owning a few great stocks; that is where all the money is. Turn it around and it becomes one of the strongest arguments for broad indexing ever produced. If roughly 4% of stocks are responsible for all the gains, and those winners are notoriously hard to identify in advance, then a concentrated portfolio is a bet that you can pick from a deck where about 96% of the cards range from mediocre to ruinous. The base rate is brutal: a concentrated portfolio is more likely to miss the few big winners than to catch them, which is why, as Bessembinder notes, poorly diversified active strategies so often underperform.

The index fund solves the problem by brute force: own everything, and you are certain to own the 4% that matter. You accept that many holdings will be duds in exchange for certainty that you’ll capture the few superstars whose returns make the whole thing work. This is the research-grounded case for indexing. It isn’t “markets are efficient”; it’s “the winners are too concentrated and too unpredictable to risk missing.” It’s also why the studies of actual millionaires find them built on boring diversified funds, not concentrated bets.

The catch: concentration cuts both ways

In fairness to the concentrators, the same data contains their defense. People who did hold the 4%, who bought and held one of the great compounders for decades, earned life-changing returns no index could match, and a handful of brilliant or lucky investors built fortunes exactly this way. Concentration is how you get spectacularly rich; diversification is how you get reliably wealthy. The honest framing is that concentration has a wildly skewed payoff: a small chance of enormous wealth and a large chance of trailing cash. Whether that is a good bet depends on whether you can identify winners in advance, and the evidence that almost anyone can do that reliably is thin; Jim Simons is the rare exception that proves how hard it is.

Are things different now? The Magnificent Seven problem

Now the modern wrinkle, and it’s a big one. The clean “the index diversifies you” story has a hole in it, because the index has itself become a concentrated bet. The Magnificent Seven (Apple, Microsoft, Nvidia, Alphabet, Amazon, Meta and Tesla) grew from about 12% of the S&P 500 in 2015 to roughly a third by 2024, and those seven names drove well over half of the index’s return in 2023. Morgan Stanley found the pace at which the largest stocks’ share of the index rose to be the fastest since 1950. When you buy an S&P 500 fund today, about a third of your money goes into seven closely related mega-cap technology companies. That is not the broad, own-everything portfolio the index-fund pitch describes. It is, increasingly, a big-tech bet dressed as diversification.

So are things different? The honest answer is more interesting than either “it’s a bubble” or “relax.” The concentration of the index is unusual: by historical standards it is extreme, and the growth of passive investing may be amplifying it, since cap-weighted index flows buy more of whatever is already biggest regardless of price. But the concentration of returns is not new at all. Bessembinder’s whole point is that a tiny fraction of stocks has always driven nearly all of the market’s wealth; every era has had its own handful. What’s unprecedented is how much of the index you hold is now those winners. What’s ancient is that the winners were always a tiny few.

What it means for the long-term investor

Put the pieces together and a clear-eyed conclusion emerges. For most people, broad indexing remains the evidence-based default, because the Bessembinder math makes stock picking a loser’s game for all but the exceptional. But it is worth dropping the comforting idea that an S&P 500 fund is fully diversified: it is substantially a bet on seven companies, and if those names fall together, so does the “diversified” portfolio. The practical questions aren’t exotic. Do you know your real exposure across every account and fund? Is any deliberate concentration sized so that being wrong can’t ruin you? And whichever path you choose, the thing most likely to hurt you is rarely the structure. It’s your behavior when the concentrated bet, index or otherwise, has its inevitable bad stretch.

From belief to behavior: concentration is a decision you keep making

Is your portfolio concentrated without you realizing?

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Resources and further reading

• The landmark study: Bessembinder, H. (2018), “Do Stocks Outperform Treasury Bills?”, Journal of Financial Economics 129(3): 440–457.
• Global confirmation: Bessembinder, H., Chen, T.-F., Choi, G. & Wei, K. C. J., “Do Global Stocks Outperform US Treasury Bills?”: the same skew across dozens of markets.
• Concentration data: Morgan Stanley research and CNBC reporting on the S&P 500’s rising top-heaviness and the Magnificent Seven’s share of index returns.
• The active-vs-passive evidence: S&P Dow Jones Indices, SPIVA Scorecard; see also Gecko on which strategies actually work.
• The case for indexing: Malkiel, B. G., A Random Walk Down Wall Street, reviewed in Gecko’s Book Notes.

Frequently asked questions

Essay in Gecko’s investing and trading psychology series. Figures are drawn from Bessembinder (2018) and market-concentration research and reporting (Morgan Stanley, CNBC) as of 2026, and are approximate. Nothing here is a market forecast or a recommendation of any security, fund or strategy; the concentration-versus- diversification choice depends on your own circumstances. Gecko is an educational and informational tool. Nothing here is financial, investment or trading advice. Investing carries substantial risk of loss.