Today’s Cap-Weighted Index Is an Identification Bet

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Most professionals already know the cap-weighted S&P 500 has disconnected from its equal-weighted fraternal twin, the S&P 500 Equal Weight Index, by the widest margin in fifty years. The gap is the result of an AI bet concentrated into a handful of names at a steep valuation. I have lived through this pattern twice before.

Start with what the consensus gets wrong: The index has grown dangerously concentrated, and equal weighting is the cure. However, the risk was never in owning a few names. It has always been in owning the particular few the market has crowned as winners, at maximum weight, on the unexamined bet that it crowned the right ones.

To be clear, the market has reliably crowned the wrong ones on prior occasions. The cap-weighted S&P 500 does not hold the eventual winners so much as presumed winners, at the largest possible weights. That is what makes it an identification bet rather than a breadth bet.

If the market has correctly named the companies that will dominate the AI era, cap weighting will look brilliant, because it owns them in size and will ride them up for free. If the market has misidentified even one or two of them, the cap-weighted index absorbs that loss at full weight, precisely where it is most concentrated. The real question is: How much do you want to bet the market chose the correct companies?

A Familiar Story Arc

This has happened twice in my career, and the arc never changes: An adoption inflection arrives, capital crowds into the presumptive winners, the divergence widens, a shakeout comes, and on the far side, a small number of names are validated while a long tail reverts. The pattern is not that everything mean-reverts; it is that the few real compounders earn their premium and the many pretenders give it all back.

In the personal-computer era, the inflection most people remember is the launch of Windows 95, the 1995 moment that a computer became something an ordinary household would buy. I watched the merciless and clarifying sorting that followed as Microsoft and Intel compounded into giants. Meanwhile, the forest of clone makers and one-product software shops that had been priced as the companies of the future mostly disappeared. As the revolution matured, most of the companies betting on it were not the winners.

The internet ran the same arc at higher amplitude a few years later, and I was inside it this time. The profitable company where I was CFO, SoftAware Networks, was acquired by Digital Island in 2000. The deal was struck almost entirely in Digital Island stock at a paper value near $450 million. I left when the deal closed, because the acquiring company had not even produced gross profits.

That proved to be a timely decision, as, within about a year, Digital Island's stock fell roughly 99%. Cable and Wireless bought the pieces at a little over $3 a share, down from a high near $57. The paper fortune was never real, because Digital Island was a pretender the market mistook for a winner. The companies that actually survived that shakeout — Microsoft (after an early stumble), Amazon and Google — are the ones still standing.

AI Makes It a Trifecta

Today, artificial intelligence is my third run of this arc, and it is still early. The adoption inflection has plainly arrived, and the divergence is at a 50-year extreme. What has not arrived is the shakeout, the event that separates the two or three companies that will earn monopoly-scale returns from the dozen being priced as if they might.

History is unkind on this count: The personal computer did not support a dozen dominant platforms, and neither did search nor social networking in the Internet Age. It is hard to see how a field as capital-intensive as AI, with network effects this strong, supports more than a few successful competitors. Who among today's leaders are the Microsofts and who are the Digital Islands remains unclear, but the cap-weighted index has bet heavily on its largest holdings.

And that bet has rarely been larger. Cap weighting has beaten equal weight by about 30% since 2023, with the 10 largest names representing nearly 40% of the index. However, over the long run, equal weight has won — by roughly a point a year since 1971.

The dominance everyone treats as the natural order is the anomaly. The mechanics only deepen it. The equal-weight index’s quarterly rebalance is a spring that loads as the gap widens. But for cap weight to keep pulling ahead, its giants must keep outgrowing an ever-smaller remainder of far more companies, which cannot continue forever.

Earnings vs. Enthusiasm

Cap weight’s success turns on one measurable question: Is the divergence built on earnings or on enthusiasm? In 2000, the 10 largest companies traded at nearly 43 times earnings against 21 times for the rest, though those top 10 were producing less than one-fifth of the market’s profits. Today, on RBC Wealth Management’s numbers, the top 10 trade at nearly 31 times earnings against 21 times earnings and produce close to one-third of market profits.

The current giants are expensive, but they are earning their keep in a way the market leaders in 2000 never did. That is the strongest case for believing that this time is different. It is also incomplete, because a premium of 31 over 21 is still a premium, and premiums are springs come rebalancing time.

The fair objection is that cap weighting never had to be right about every company, only about enough. After all, the gains from the names it gets right can outrun the losses from the ones it gets wrong. That asymmetry is thinner than it sounds: If you held the NASDAQ index at its 2000 high, it took fifteen years to recover fully from the reset. Today’s contenders offer both a lower premium and greater earning power, so that same event today is most likely less deep.

Unfavorable Odds

Still, the hoped-for skew runs against the optimist, because a tiny fraction of stocks produce nearly all the net wealth. The winners must not only win, they must already sit in the index at the largest weights to drag a long tail of losers behind them when the reset happens.

If the next compounder is still private, or too small to register, or simply not yet the name the market has crowned, cap weighting cannot ride the stock up to pay for the index’s mistakes. The math that is supposed to rescue cap weighting depends on owning the winners early and cheaply — the one thing a portfolio built entirely on yesterday’s prices cannot do.

For an advisor, the practical implication is a disclosure. A client who owns the cap-weighted index owns an identification bet on the current leaders. That bet should be sized on purpose rather than inherited by default.

Equal weighting deserves a more precise defense than the usual one. Identification-risk diffusion is a way of declining to bet that the consensus has the names right. You can believe the technology changes everything and still decline to bet a client's retirement on the market's current guess about who captures it.

An Uncertain Outcome

Where do I land? The evidence is split, and I have tried to present both halves fairly. The earnings backing today's giants are real and far sturdier than in 2000, which is why this is not a simple replay. But the premium is still a premium, the rebalancing spring is loaded, the concentration sits at an arithmetic extreme, and the shakeout that sorts the real winners from the priced-in pretenders has not yet landed.

The same shakeout that came for the clone makers and Digital Island will come for whichever of today’s giants the market has misnamed, because some winners might not yet be publicly traded or in any index. This time will be different for only the two or three names that prove real — it will rhyme for everyone else.

John Templeton labeled “this time is different” the most expensive phrase in investing. The cap-weighted index has bet that it can already tell the survivors from the pretenders, but history says it cannot. So the only question left is whether you want that bet made on your behalf, at maximum size, before the sorting begins.

Notes and sources

Three-year and long-run cap-weight versus equal-weight returns: S&P Dow Jones Indices data as reported in market coverage; the roughly 30% gap since the start of 2023 surpasses the dot-com peak, and the long-run equal-weight edge of about a point a year dates from 1971.
Index concentration, top 10 near 40% of weight (40.7% at year-end 2025) against about 32% of earnings, and cap-weight trading at roughly a 30% premium to equal weight: RBC Wealth Management, “The Great Narrowing,” FactSet data as of December 31, 2025; Magnificent Seven near 34 percent: Lord Abbett and S&P Dow Jones Indices data.
Earnings versus multiple comparison with 2000, top-10 forward P/E of roughly 31 versus 21 today against 43 versus 21 in 2000, and earnings share near 30% versus under 20%: Goldman Sachs, data as of August 2025. Multiple is stated on a forward basis; trailing or simple-average measures of the top 10 run higher.
Concentration of stock-market wealth in a small fraction of names: Hendrik Bessembinder, "Do Stocks Outperform Treasury Bills?", Journal of Financial Economics, 2018.
Digital Island sale to Cable and Wireless at about 3.40 per share, down from a 52-week high near 57.37: company and press reports, 2001.

More articles by Mark Tennenbaum:

Mark Tennenbaum is the founder and chief investment officer of Life UnLocked Partners, a California-registered investment adviser, and chief executive of Leyland Cypress. He began his career building securities-pricing software in the City of London, Britain’s Wall Street equivalent, later served as chief financial officer of one of Los Angeles’s first commercial datacenters and of SoftAware Networks, and co-founded a messaging-security firm acquired by Microsoft in 2005. His research on measuring active manager skill is under peer review at the Journal of Portfolio Management.

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