After the Fed hike, 3,480 stocks rose. The S&P 500 still needed chips

The eight largest positive contributors produced two-thirds of Thursday's S&P 500 gain, even as most U.S. stocks finished higher. The count and the cap-weighted index finally pointed the same way. The dollars did not spread out with them.

At the 4 p.m. Eastern close, 3,480 stocks advanced, 2,092 declined and 251 were unchanged among 5,823 U.S. common stocks listed on the NYSE, Nasdaq and NYSE American (ETFs, funds, foreign listings, FX and crypto symbols excluded). That is a 59.8% advance share, a day after the Federal Reserve raised rates for the first time since 2023 and most names fell. The S&P 500 fund ASPY still gained more than twice as much as the equal-weight S&P 500 fund BRSP: 1.13% against 0.49%, a 0.65 percentage-point gap. The Nasdaq-100 fund AQQQ rose 1.73%. The Russell 2000 small-cap fund AIWM rose 0.51%.

Reuters, writing Thursday, tied Wall Street's bounce to easing oil prices, dropping Treasury yields and solid labor data. The Labor Department reported that initial jobless claims fell by 10,000 to 196,000 in the week ended September 12. West Texas Intermediate crude fell 1.1% to $101.34 a barrel, and the 10-year Treasury yield was 4.94%.

Four semiconductor stocks produced half of the Nasdaq-100 gain

Micron, AMD, Nvidia and Intel contributed 0.88 percentage points, 51% of AQQQ's 1.73% gain. 24/7 Wall Street, writing Thursday, linked Intel's move to reports of early talks with SK Hynix to produce memory chips at Intel's Ohio complex, and described a third consecutive session of semiconductor gains. That was not a uniform bid for large growth: T-Mobile was the worst drag in the Nasdaq-100 fund, and Netflix also fell.

Chips did the sharp work in the S&P 500 fund too. Nvidia and Micron sat among ASPY's four largest contributors, together with Apple and Microsoft, for 0.48 percentage points of the move. The eight largest positive contributors accounted for two-thirds of it. Nvidia, an 8.0% weight, rose 2.5% and did the most work; Apple added 1.4% and Microsoft 1.5%. The fund's ten largest positions are 38.2% of assets, so a handful of names can still dominate a day on which thousands of stocks rise.

Fourteen of 20 mega-cap stocks rose, including Nvidia, Apple, Microsoft, Amazon (up 2.1%), Broadcom (2.3%) and Tesla (2.3%). The six that fell were Berkshire Hathaway Class B, off 2.0% and the largest named drag on ASPY; Netflix; Walmart, down 0.7%; Mastercard and Visa, each slightly lower; and Exxon Mobil, which finished barely down. Those six did not offset the chip and mega-cap gains in the cap-weighted funds.

Equal-weight had no such engine. Moderna was BRSP's top contributor at 0.05 percentage points. Eight names together accounted for only about a third of that fund's gain, which is what equal weight is built to do: it rose because hundreds of S&P 500 stocks rose a little, not because a few rose a lot.

Equal-weight S&P 500 is still below its 50-day average

A majority-up session is the textbook picture of a broad market. The funds that hold the S&P 500 told a more concentrated story.

That concentration is the risk if the chips stop leading. In his June 30, 2026, Global Market Structure and Flows review, Citadel Securities' Scott Rubner put the ten largest companies at nearly 40% of the S&P 500, near record levels, and argued that anyone holding a broad index fund was carrying more semiconductor risk than the label suggests. Thursday was a session of that structure: four chip stocks produced half the Nasdaq-100 gain.

ASPY closed at $762.60, above its 50-day average of $759.53; Wednesday's close was $754.05, so the bounce carried it back through that line. BRSP closed at $213.31 against a $217.27 average and never got there. AIWM remains further below its own 50-day line. Year to date the two S&P 500 funds are still nearly even, up 12.4% and 12.3%. The split is recent, and Thursday did not close it.

If you hold the cap-weighted S&P 500, Thursday looked like a recovery. If you hold equal-weight, it was a partial refill.

Across Wednesday and Thursday together, ASPY is up 0.69% and BRSP is still down 0.30%. The Nasdaq-100 fund gained 1.76% over those two sessions. The small-cap fund gained 0.08%, essentially unchanged through the hike and the bounce.