Wall Street has a market paradox on its hands. The S&P 500 has climbed roughly 14% in 2026 and reached record levels, yet more than 40% of the stocks inside the index have declined this year.

That disconnect is raising an important question for investors: Is the U.S. stock market genuinely getting stronger, or are a handful of AI winners making the entire market look healthier than it really is?

According to a Reuters analysis published Monday, the advance has been driven disproportionately by a relatively small group of AI-related companies and semiconductor manufacturers. Meanwhile, the median S&P 500 stock has gained only about 4% this year.

The distinction matters because index performance can conceal considerable weakness beneath the surface.

Companies connected to artificial intelligence have attracted substantial investor attention as demand for advanced computing, data centers and memory chips continues to reshape technology spending.

Shares of Sandisk and Micron Technology have more than tripled this year, according to Reuters, while Nvidia remains a major participant in the AI investment story.

These gains have helped push the broader benchmark higher. However, when a relatively small number of powerful stocks account for a large share of index performance, investors become more dependent on those companies continuing to deliver exceptional results.

A disappointment from several major AI stocks could therefore have a disproportionate effect on the broader market.

There is another warning sign: Fewer than half of S&P 500 constituents were trading above their respective 200-day moving averages, according to the Reuters analysis. That suggests the rally has not been accompanied by equally strong participation across the index.

A narrow rally does not automatically signal an impending crash. Markets can remain concentrated for extended periods, particularly when a dominant technology trend produces exceptional earnings growth.

But concentration changes the balance of risk.

Investors should examine whether their portfolios contain several companies exposed to the same underlying AI spending cycle. A portfolio holding a chipmaker, a cloud provider and a data-center infrastructure company may appear diversified while still depending heavily on the same investment trend.

The other question is valuation. Even companies with compelling long-term prospects can experience sharp declines if their share prices already reflect years of expected growth.

Investors looking for evidence of a healthier market should monitor whether participation broadens into financials, industrials, consumer companies and other sectors.

First, watch upcoming earnings reports for evidence that AI-related revenue growth remains durable rather than simply reflecting elevated expectations.

Second, monitor how many S&P 500 stocks participate when the index rises. Broader participation would provide stronger confirmation of the rally.

Finally, keep an eye on Treasury yields. Higher borrowing costs can pressure expensive growth stocks and raise the bar for future returns.

The S&P 500's record performance tells only part of the story. Beneath the headline number, a large share of listed companies is struggling to keep pace.

For investors, the lesson is not necessarily to abandon stocks. It is to look beyond the index, examine concentration risk and distinguish companies delivering durable earnings growth from those benefiting primarily from market enthusiasm.

The next phase of the rally may depend on whether Wall Street's biggest winners can continue outperforming—or whether the rest of the market finally catches up.