The US stock market's bull run is approaching its fourth anniversary, and the S&P 500 is once again flirting with record highs. According to Reuters, the index is hovering near its all-time peak heading into October 12, powered by a wave of corporate spending on artificial intelligence that has translated into stronger earnings.
But the rally is not without its stresses. Rising Treasury yields and an unusually heavy concentration of market value in a handful of mega-cap technology names are raising questions about how much longer the good times can roll.
From hype to hard cash
The AI story has evolved from buzzword to balance sheet reality. Big Tech "hyperscalers" such as Microsoft and Alphabet have poured billions into data centers and specialized chips, and that capital expenditure is now showing up in their bottom lines. Analysts' forecasts call for S&P 500 earnings to climb more than 35% this year, a pace that would be remarkable in any environment.
Oxford Economics estimates that roughly one-third of recent US economic growth can be tied back to AI, both through direct investment and through a "wealth effect"—the idea that rising stock prices make households feel richer and more willing to spend. That spending, in turn, feeds back into corporate revenues.
For everyday investors, the takeaway is that AI is no longer just a theme for speculative traders. It is a genuine driver of profits across the index. But that also means the market's fate is increasingly tied to the fortunes of a relatively small group of companies.
A narrow rally
The problem is that the bull market has been anything but broad. Technology and communication services sectors have done most of the heavy lifting, while many other industries have lagged. J.P. Morgan Asset Management data cited by Reuters shows the top 10 stocks in the S&P 500 now account for about 40% of the index's total value, up from roughly 28% in October 2022.
That concentration makes the benchmark more sensitive to the fortunes of a few AI-linked giants like Nvidia, Alphabet, and Meta Platforms. When those stocks move, the whole index moves with them—sometimes dramatically. As Treasury yields hover near multi-decade highs, the risk is that a rate-driven selloff in mega-caps could drag down the entire market, even if the average company is doing just fine.
The yield problem
The backdrop is also less forgiving than it was a few years ago. The 10-year Treasury yield is hovering near 5.2%, after recently touching a 24-year high. Higher "risk-free" yields raise the discount rate investors use to value future profits, which makes stocks—especially growth stocks that promise big earnings far in the future—less attractive by comparison.
When Treasuries offer around 5.2%, stocks have to work harder to justify their prices. Investors can earn a solid return without taking on equity risk, so the extra return they demand for owning stocks—the so-called equity risk premium—tends to compress. That can show up as lower valuations unless companies keep beating earnings expectations.
As high yields cap gains in other markets, the pressure is not unique to the US. But the S&P 500's heavy reliance on a few mega-caps amplifies the effect. A rate-driven reset in those stocks can make the headline index unusually sensitive to both yields and the next round of AI earnings proof points.
What it means for investors
For the average investor, the key takeaway is that the bull market is real but increasingly fragile. The AI-driven earnings boom is a solid foundation, but it is built on a narrow base. If the top 10 stocks stumble—whether because of disappointing AI results, a spike in yields, or simply profit-taking—the index could feel it more than in past cycles.
Diversification matters more than ever. Investors who are heavily weighted in tech or in the S&P 500 as a whole should be aware that their portfolio's performance is increasingly tied to a few names. That doesn't mean abandoning stocks, but it does suggest paying attention to how much of your exposure is concentrated in the AI trade.
Also worth watching: the path of Treasury yields. If they keep climbing, the pressure on valuations will intensify. If they stabilize or fall, the bull market could have more room to run. As US yields dip slightly in some sessions, traders are clearly watching for any sign of relief.
The next few weeks will be telling. With earnings season underway, investors will be looking for confirmation that AI spending is still translating into profits—and that the rally's leaders can keep delivering. Until then, the four-year-old bull market remains a powerhouse, but one that is increasingly walking a tightrope.
