Wall Street outperformed September's historical pattern despite rising Treasury yields, with artificial intelligence (AI) stocks driving a sharp divide among major indexes. The S&P 500 fell 0.4% during the month and remained 1.9% below its August record, compared with an average September decline of 0.6% since 1950. The Nasdaq Composite gained 1.8% after reaching a new peak on Sept. 22, helped by Meta, AMD and several semiconductor stocks. The Dow Jones Industrial Average dropped about 4.3%, while the Russell 2000 lost 5.4%. Interactive Brokers chief strategist Steve Sosnick described the market as "AI or bust," while cautioning that underlying conditions were weaker than the headline index performance suggested. Two-year and 10-year Treasury yields each rose roughly half a percentage point, and the 30-year yield increased nearly 40 basis points from the end of August. The last comparable move across all three maturities occurred in September 2022, during the Federal Reserve's previous hiking cycle. Tighter financial conditions have also pressured the initial public offering (IPO) market. Smart-ring maker Oura postponed its planned offering, following Holtec Nuclear and Bamboo Insurance, which also cited uncertain market conditions. Anthropic reportedly delayed its planned public debut from October to November, making the frontier AI company's listing an important test of investor appetite for the sector. UBS chief economist Arend Kapteyn said AI investment and the wealth effect from rising AI stocks now account for 80% or more of U.S. economic growth. UBS raised its estimate for AI-related capital spending next year by $200 billion to $1.4 trillion, while non-AI component investment is "running at zero." Kapteyn said the concentration creates risks for both business investment and consumer spending if AI stocks reverse, particularly because higher-income households have benefited from the wealth effect. UBS also sees evidence that the boom is spreading beyond technology: each dollar of AI-related capital spending this quarter appears to generate about $1.50 of investment in the following quarter, with roughly one-third of that follow-on spending directed toward technology.