What happened

The S&P 500 is the main index used to track large U.S. stocks. A recent MarketWatch report says the index has become a poor guide to the average stock. On Monday, the S&P 500 finished only 0.3% below its latest record close in August. Yet the median S&P 500 stock stood about 17% below its 252-day high, according to Warren Pies of 3Fourteen Research. The headline index and the middle stock were telling very different stories.

Why the gap exists

The S&P 500 uses market-cap weighting. Larger companies have a larger effect on the index. This makes the index heavily sensitive to a small group of giant firms. The group known as the Magnificent Seven includes Microsoft, Apple, Nvidia, Alphabet, Meta Platforms, Amazon, and Tesla. Together, these companies ended Monday with a market value just below $25 trillion. That was a new record, according to the report.

The growing influence of artificial intelligence has helped separate these firms from many other stocks. Strong interest in AI can lift the biggest companies. Their gains can then keep the S&P 500 near a record, even when many other companies are weaker.

Why it matters

This difference is called weak market breadth. Breadth asks whether a market rise includes many stocks or only a few leaders. The report says the three-month relationship between the market-cap-weighted S&P 500 and its equal-weight version was moving toward one of its lowest recent levels. That suggests the index and the average company were moving less alike.

For investors who own fewer shares of the biggest winners, this can create a misleading view of market health. A strong index does not automatically mean that business conditions are improving across most companies. It also means that a setback among the leaders could affect the headline index quickly.

What analysts see

Goldman Sachs derivatives strategist Brian Garrett said the S&P 500 no longer behaves like the main clearing price for risk in the U.S. market. In simpler terms, he believes the index has become less connected to the wider market. Ari Wald, head of technical analysis at Oppenheimer, described the break between the index and the average stock as late-cycle behavior. The number of Russell 3000 stocks above their 200-day moving average had also fallen sharply. That pattern usually appears when major indexes are struggling.

What remains uncertain

Narrow breadth is a warning sign to monitor, not a guaranteed forecast. During the dot-com boom, the New York Stock Exchange advance-decline line peaked in 1998. The S&P 500 and Nasdaq still climbed for about two more years. The current AI-led rally could also keep major indexes rising.

The latest picture is mixed. Breadth improved over the most recent two trading sessions. Ten of the S&P 500's eleven sectors rose for a second straight session. The important question is whether that improvement lasts.

What to watch next

The next signal is whether strength spreads beyond AI-related giants. Investors can compare the S&P 500 with its equal-weight version. They can also watch the number of stocks above their 200-day averages, credit spreads, and hedge-fund leverage. These measures will not predict the future alone. Together, they show whether the market's strength is broadening or staying concentrated in a few powerful names.