The S&P 500 Stopped Being the Economy, and Nick Colas Just Said It Out Loud

I keep a running list of market assumptions that need to die. Near the top sits the idea that the S&P 500 is the economy.

Nick Colas took a swing at that one on Thursday. The DataTrek co-founder told CNBC'sClosing Bell Overtimethat tech and big tech now account for roughly 50% of the S&P 500. Half the index. He argues that makes the benchmark a weaker proxy for US economic activity than it has ever been.

The timing is telling. The 10-year Treasury yield slipped below 5% the same day. Bulls read the drop as relief. Bears read it as growth cracking. Colas points to AI funding fears easing as the driver. Here's what matters: the same handful of companies driving the index are also driving the yields that supposedly explain the index.

The Mechanics of a 50% Index

Let me break this down. Market cap weighting doesn't care about the economy. It cares about size. When one chipmaker adds a few hundred billion in market value, every passive dollar gets funneled into it whether the buyer knows it or not.

Two decades ago, the tech sector's weight was closer to the mid-teens. Today the top ten names carry roughly two-fifths of the index, and the big-tech cluster lands near the 50% mark Colas cites. A regional bank, a railroad, and a homebuilder are now rounding errors in a benchmark that pension funds treat as a proxy for everything.

So what are you actually buying when you buy an S&P 500 index fund? Mostly a bet on AI capex continuing. That's not a knock on AI. It's a description of the exposure.

Why the Signal Is Getting Noisier

Yields below 5% used to be a macro story. Fed path, inflation, growth. Now a big chunk of the move comes from how investors feel about financing enormous data center buildouts. These are capital intensive projects, and plenty of them lean on debt markets. When funding fears ease, yields drift lower. When they flare up, yields climb. The index and the bond market end up trading off the same AI headline.

From a risk perspective, that's a concentration problem dressed up as a macro indicator. Anyone using the S&P 500 to forecast GDP, consumer spending, or hiring is reading the wrong tape. Small caps and equal-weight indices are the honest tell. They've lagged for a reason.

And regular people feel this. A 401(k) parked in a cap-weighted index fund is now a concentrated tech bet with a comforting name. That's fine while the trade works. It's less fine when it doesn't.

My Take

I think Colas is understating it. Fifty percent isn't a quirk of weighting. It's a structural change in what public markets represent. The US economy is services, healthcare, housing, logistics, and government. The index is software, semiconductors, and platforms.

So stop asking the S&P 500 how the economy is doing. Ask it how AI capex is doing, then go look somewhere else for the economy.

What I'd watch next: whether the 10-year holds below 5% on real data rather than sentiment, whether equal-weight starts closing the gap, and whether breadth improves into year-end. Breadth is the tell for conviction versus flows.

The numbers tell the story. Half the index isn't the economy. It never was, and now the gap is too wide to ignore.