US equity investors have a lot to cheer of late, despite stubborn inflation, rising interest rates and financial tightening, and falling consumer confidence. The S&P 500 has hit 28 closing highs this year, and the index is up 103 percent since the start of 2023 — slightly more than doubling even excluding dividends. While one would be foolish to predict that a stock market correction or crash is necessarily imminent, record levels in a given stock index can be misleading. Indices can be structured in ways that create the outward appearance of market health despite lackluster performance, and even sagging fortunes, beneath the index value. In early October, only an estimated 25 percent of stocks making up the S&P 500 were trading above their 50-day moving average. By the end of Monday, October 12, that number had fallen to 21 percent. 

Per Bloomberg analysis, over the past three months, the ten largest S&P 500 stocks have gained 7.06 percent; the entire equal-weight S&P 500 index has lost 1.49 percent. That is an outperformance of 8.55 percentage points, putting the current reading among the most extreme episodes of megacap leadership in Bloomberg’s historical records.

An index can rise even when many, even most, of the stocks within it are declining in price. In a capitalization-weighted index (see holdings, here), bigger companies account for more weight. An equal-weighted index, on the other hand, gives each stock the same weight (as shown here) regardless of size. The S&P 500 index customarily reported in the financial news media uses a capitalization weighting based on the shares available for public trading and the stock price. A company with a float-adjusted market value of $2 trillion in an index totaling $50 trillion would therefore have a 4 percent weight. In an equal-weighted index with 500 constituents, that same company would have a 0.2 percent weight — as would each of the other 499.

The number of stocks rising versus falling in a particular index is known as the market “breadth;” tracking that number helps distinguish a broad advance in stock prices from one carried by a small number of companies. In a capitalization-weighted index, narrow breadth can easily lead  market conditions to appear healthier than they are. 

The S&P 500 index which is typically reported in the financial news is the cap-weighted index. On October 12, 2026, Reuters reported that the index was up just over 14 percent year-to-date. But beneath the surface, at least 200 of the 500 constituent stocks are down for the year, and the median constituent is up 4 percent. The worst-performing quarter of the cap-weighted index constituents are down at least 11 percent, though. And more than 10 percentage points of the index’s 14 percent gain has come from a handful of major tech stocks within the index.

In a cap-weighted index, larger companies have a substantially greater impact on the index than smaller names. In the case of the S&P 500, presently, roughly ten companies compose about 40 percent of the total index weight; three years back, the top ten companies constituted roughly 28 percent of the index. This underscores several considerations: first, that while diversification is usually sought to spread out the risk among a large number of assets in a portfolio, market-cap weighted indices are more diversified by individual stock names than the risks that drive stock prices. 

An advance-decline line, here provided by Bloomberg for the S&P 500, tracks the cumulative daily difference between rising and falling stocks; a rising number depicts an increasingly small number of index constituent stocks contributing to the index’s gains.

For that reason, the type of index that investors own may impact the extent to which their overall risk is spread. When the stocks of ten firms account for 40 percent of an index, their performance vastly outweighs that of the smaller index holdings — both up and down. Thus while nominally diversified across 500 stocks, the risks that directly impact the largest index constituents — in this case, technology-specific risks, such as expectations for technology earnings and  AI regulation — will disproportionately impact the index returns. And that concentration can follow investors across numerous accounts: a 401(k), IRA, and separate mutual funds may each follow capitalization-weighted methodologies, creating numerous lines of narrow exposure to a small handful of publicly-traded companies.

None of those prominently represented firms has to fail outright for investors’ exposure to become costly. Indeed, throughout history, businesses have seen growing revenue and profits even as their share prices fell. The key question is the multiple that investors are willing to pay for earnings. Any disappointment over the potential for AI firms to recoup the massive capital expenditures they’re undertaking, or a threat posed by competitors or new technologies, could substantially change investors’ views of firms and the sector at large. And in that case, a few falling stock prices could drag down a capitalization-weighted index, even while the companies behind those equities remain profitable.

There is a wide gulf between dependable firms and dependable investments. In the 1960s, the “Nifty Fifty” — a loosely defined basket composed of the stocks of rapidly growing US companies — were deemed so reliable that they were viewed as a buy at virtually any price. By the early 1970s their valuations had become exceptionally lofty, and in the 1972 to 1974 bear market the equal-weighted Nifty Fifty composite declined by more than the S&P 500’s 48 percent decline. During the dot-com collapse from March 2000 to October 2002, the S&P 500 index declined 49 percent. The index was not completely composed of internet stocks, nor were the technological importance of those firms imaginary, but the prices investors paid throughout most of the period were not justified by the earnings that materialized.

Of course, the concentration of an index says nothing of either timetables or eventualities. In 2023, the cap-weighted S&P 500 returned just over 26 percent to investors, compared with just under 14 percent for the same, equal-weighted index. The leadership of a given index can narrow as surely as it broadens, and investors’ responses to earnings, macroeconomic developments, and a wide variety of other inputs ultimately indicate what outcome lies beyond the next quarterly earnings report or Fed announcement. 

(Returning to the concept of breadth here, the thoughtful investor may ask: is narrowing breadth amid a rising market a harbinger of poor performance? Again — while history does not dictate future outcomes — that question has been analyzed on several fronts.)

It’s not that indices reaching record highs are meaningless, nor that sticking with equal-weighted indices necessarily ratchets down risk. A headline index, whether it’s the S&P 500, the Dow Jones Industrial Average, or the Nasdaq 100, describes a particular portfolio with its own nuances. A rise in any one index neither establishes the health of American businesses, nor guarantees that investors whose portfolios track that index are meaningfully diversified.