Jeremy Grantham has seen a few market manias. The 1987 crash. The dot-com blowup. The housing bubble. And now, he says, we're in another one — maybe the granddaddy of them all.
“This is the most expensive market in American history,” Grantham tells CNBC, his voice flat, almost weary. He's not talking about inflation-adjusted PE ratios or some obscure metric. He means it. By his reckoning, the U.S. stock market has never been this overpriced, not even in 1929, not even in 2000.
The AI Fuel Injection
The culprit? Artificial intelligence. The same technology that's supposed to transform everything has transformed stock valuations into something unrecognizable. Grantham points to a handful of mega-cap tech stocks — the usual suspects — that have sucked up all the oxygen. They trade at multiples that would make a growth stock blush, and investors keep piling in, convinced this time is different.
It's never different. Grantham has made a career betting that gravity eventually wins. He co-founded GMO, the asset management firm that's built a reputation for calling bubbles early and often. In 1999, he warned that tech stocks were in a bubble. He was right. In 2007, he warned about housing. Right again. Now he's waving the red flag on AI.
“The market is priced for perfection. And perfection is a rare thing.” — Jeremy Grantham
His logic is brutal but simple: the AI boom has inflated a bubble in a handful of stocks, dragging the entire market to nosebleed levels. The S&P 500 looks expensive, but strip out the top ten names, and it's closer to fair value. That's not a healthy market. That's a house of cards.
History Repeats, But Not Exactly
Grantham isn't the only one hearing echoes of the late 1990s. Back then, the Nasdaq was the playground of dreamers. Today, it's AI. Same story, different acronym. But there's a twist: this time, the bubble isn't spread across hundreds of dot-com startups. It's concentrated. That makes it both more fragile and more dangerous.
“The concentration of market cap in the top few stocks is unprecedented,” Grantham says. Apple, Microsoft, Nvidia, and a few others now account for a staggering share of the S&P 500's total value. If those stocks sneeze, the whole market catches pneumonia. And sneeze they will, because gravity doesn't take a holiday.
The question isn't whether AI will change the world. It probably will. The question is whether investors are paying 30–40 times future earnings for companies that might not deliver for a decade. Grantham's answer: hell no.
What Comes Next?
Grantham doesn't predict timing — he's not a market timer, he says — but he sees the math. The S&P 500 is trading at about 40 times trailing earnings. That's double the historical average. Even if you adjust for low interest rates, it's still rich. And with rates now higher than they were a few years ago, the case for stocks gets even thinner.
“People are behaving as if the world will be wonderful forever,” he says. “That's not how it works.”
He points to the bond market, which is offering real yields that are actually attractive for the first time in years. Why would anyone take the risk of stocks at these levels when they can get 5% from a Treasury? The answer, he says, is FOMO. The fear of missing out is driving money into stocks that have already run up 50%, 100%, 200% or more. That always ends badly.
The Bear Case for AI
Let's be clear: the AI revolution is real. Language models, image generators, autonomous systems — they're changing industries. But the euphoria has disconnected stock prices from business reality. Most AI companies aren't profitable. Many will never be. The ones that are — like Nvidia — have seen their stock prices triple in two years, pricing in a future where AI chips fly off shelves forever. That's a lot to ask.
Grantham compares it to the early days of the internet. Yes, Amazon and Google eventually dominated. But they were rare exceptions. Most dot-com stocks went to zero. The same will happen with AI. A few winners will emerge, but the herd will get slaughtered.
“Investors are treating every AI company like it's the next Google,” he says. “Most of them will be failures.”
It's not a popular message on Wall Street right now, where AI is the only game in town. But Grantham doesn't care about popularity. He cares about math. And the math says this market is a ticking time bomb.
The Verdict
So what should an investor do? Grantham is famously cautious, but he's not saying sell everything. He's saying be realistic. Don't chase the hot stock. Diversify. Have cash. Wait for the reset that always comes.
“The most dangerous words in investing are 'this time is different,'” he says. “They never are.”
The market will eventually correct. It might be next month or next year. But when it comes, the pain will be sharp. And the investors who ignored Grantham's warning will once again learn the hardest lesson: trees don't grow to the sky.
— Michael Thorpe
