AI Stocks Swung Hard in October: What It Means for Index Fund Investors

Key takeaways

• In mid-October, AI and chip stocks fell sharply on worries about how the AI build-out is being financed. In the week through October 15, 20 of the largest AI and chip stocks lost more than US$1 trillion in combined value (CNBC).
• Despite that, the S&P 500 stayed close to its record: it ended Tuesday, October 27 at 7,722.60, within about 1.2% of its early-October high (AP).
• The reason is breadth. Yahoo Finance calculated that the average S&P 500 stock beat the index by about 4 percentage points in October, on track for the biggest one-month reversal in favour of the average stock since at least 1990.
• The lesson for everyday investors is about concentration: a broad index fund can be more tied to a few large companies than it looks.

AI-related stocks have been a major part of this bull market, so when they wobble, people notice. October has been a good case study. This article explains what happened, why the broader market held up, and what it means if you own an index fund or ETF.

What happened

CNBC reported on October 15 that 20 of the world's most valuable AI and semiconductor stocks had lost more than US$1 trillion in value in that week alone. Morningstar's weekly summary put the global semiconductor index down 5.8% for the week through Thursday, with Nvidia down 5.2%, Taiwan Semiconductor Manufacturing down 8.4% and Broadcom down 7.8%. Individual chip stocks in South Korea and Japan fell 10% or more in a day.

The reasons, according to CNBC, were a mix of higher borrowing costs, worries about how the data-centre build-out is paid for, and concerns about a future oversupply of memory chips. The chief executives of Broadcom and Intel warned that financing could be a major obstacle for AI expansion. Shares in the data-centre company CoreWeave plunged on concerns about its debt, falling 25% in one week, CNBC said later.

Why financing matters

For years, the big technology companies paid for AI spending out of the cash they were earning. CNBC quoted TS Lombard economist Dario Perkins saying that debt is now the marginal source of funding, and that AI companies are expected to issue about US$700 billion of new bonds in 2027. When borrowing is the main source of funds, higher interest rates matter more, and so does what lenders think of the risk. The same article noted that business results for many AI firms still look strong: Meta reported 26% revenue growth for the quarter, and FactSet expected profits for the "Magnificent Seven" group to have grown 33% in the latest quarter.

The rebound and the swings

Volatility ran in both directions. On Thursday, October 22, the Nasdaq had its biggest one-day drop since June, according to Reuters. Then chip stocks bounced. Yahoo Finance reported that 57 of 60 stocks in its semiconductor basket rose on Monday, October 26. On Tuesday, the Reuters market wrap said the PHLX chip index (SOX) rose 3.6% and the Nasdaq 1.2%, helped by renewed AI enthusiasm after Anthropic filed its public IPO prospectus on Monday. Even so, Yahoo Finance said the SOX was still down about 12% for October as of Monday's close.

Why the S&P 500 held up

If chips fell so much, why is the S&P 500 only about 1.2% below its high? Because other parts of the market rose. Yahoo Finance reported that, for October through Monday, energy was up 13% and consumer staples up 9%, led by the best month for staples in nearly five years. The equal-weighted S&P 500, where every company counts equally, had trailed the regular index by 4.5 percentage points in September and was beating it by about 4 points in October.

Index type	How it works	October story
Market-cap weighted (the usual S&P 500)	Bigger companies have a bigger influence on the result	Held back by chip stocks but cushioned by gains elsewhere
Equal weighted	Each company counts the same	Pulled ahead in October as the average stock beat the big names

What it means for everyday investors

• Index funds are not always evenly spread. In a market-cap weighted fund, a small number of very large companies can drive much of the result. Checking a fund's top holdings shows how concentrated it is.
• One month is a small sample. The October swings are large, but one month does not say how AI stocks will do over years.
• Canadian angle. Many Canadian portfolios hold US or global index funds in a TFSA or RRSP, so US tech swings reach them even without owning an AI stock directly. The Canadian dollar can also add or subtract: it was 71.24 US cents on October 27.
• Diversification is what limits surprises. Mixing regions, sectors and asset types does not stop losses, but it reduces how much one theme can dominate the outcome.

What's next

Microsoft and Alphabet report third-quarter results after the close on Wednesday, October 28, and Amazon on Thursday. Reuters said that investors are watching cloud growth and how fast the spending turns into profit.

Sources

• CNBC: Nvidia, AMD, Dell sink in global tech rout on AI funding worries (Oct. 14–15, 2026)
• Morningstar: Markets Brief: Chip stocks sell off, investors rotate into value (week of Oct. 12–16, 2026)
• CNBC: AI credit risk grows as debt finances the infrastructure build-out (Oct. 22, 2026)
• Reuters via The Globe and Mail: Stocks lower to end a rocky week, oil prices continue to climb (Oct. 23, 2026)
• Yahoo Finance: Big Tech just went from crushing the average stock to losing to it, Chart of the Day (Oct. 27, 2026)
• Yahoo Finance: Stock market today, Oct. 27, 2026
• Reuters via Investing.com: Trading Day: AI wave lifts all boats (Oct. 27, 2026)
• AP News: Broadcom and other AI stocks lift Wall Street closer to its record (Oct. 27, 2026)

This article is for general information only and is not financial, tax or legal advice. It does not recommend buying or selling any investment. Market figures were last reviewed on October 27, 2026 and may have changed since.