Through most of the third quarter, share prices moved without clear direction — a brief upturn in mid-August having run out of steam — as enthusiasm for artificial intelligence spending cooled and elevated borrowing costs weighed on sentiment across financial markets. Equity markets had drawn notable support from the artificial intelligence sector over a span exceeding three years. Those shares lost momentum as a number of AI executives and researchers publicly raised concerns about safety risks tied to the technology's pace of development.

Semiconductor and memory shares, which had posted some of the broader market's strongest returns through the first six months of 2026, ranked among the steepest decliners over the course of the quarter. Four chip-equipment makers — Applied Materials (AMAT), Lam Research (LRCX), KLA (KLAC), and Broadcom (AVGO) — ranked among the stocks that weighed most heavily on the US Total Market Index's overall return.

A 1.36% gain was recorded by the Morningstar US Market Index during the period under review, following a 15.65% advance in the prior reporting window. Growth that had built up over the prior quarter lost momentum as the third quarter unfolded. Equity growth categories edged ahead of value-oriented holdings across the period, even as artificial intelligence stocks delivered an uneven quarter, while the Morningstar Wide Moat Composite Index posted a 4.91% gain — roughly twice what the broader market recorded.

Toward the close of September, Meta Platforms' newly launched AI agent Muse helped lift equities across the broader artificial intelligence space. Software stocks stood apart from the broader decline, posting notable gains through August after spending much of the year trailing other market segments. Shares in software companies — assets that some investors had regarded as casualties of the artificial intelligence wave — found renewed backing as a run of stronger-than-expected earnings came in and the anticipated collapse of subscription-based business models failed to materialise. Shares of Microsoft — listed on exchanges under the ticker MSFT — posted a gain of just over 33% across the three-month period.

Government borrowing costs climbed to levels not recorded in decades, as bond traders priced in a combination of widening fiscal deficits, persistent inflationary pressure, and the continuing effect of the Iran conflict on crude oil markets. Yields and bond prices run in opposite directions, and across the quarter every major Morningstar bond index posted a loss.

Swings in daily equity trading remained tied to movements in energy costs, with West Texas Intermediate crude pushing past $107 per barrel around mid-September as hostilities in Iran flared once more. Petroleum costs have been the primary force pushing broader price levels upward in the period following the outbreak of conflict, though economists note that measurable knock-on effects across other sectors of the economy have remained largely absent.

Bond markets weathered a string of sell-offs throughout the third quarter, with Treasury yields climbing to multi-decade highs on the back of four converging pressures: inflation that resisted policy efforts to contain it, an economy that outperformed many forecasts, government deficits that continued to widen, and a wave of new debt tied to the rapid buildout of artificial intelligence infrastructure. Borrowing costs tied to the 10-year US Treasury note — a rate that shapes what businesses and households pay to borrow — climbed from 5% in mid-September to a peak of 5.29% by week's end. At a point roughly half a year back, the rate on 10-year bonds sat in the neighbourhood of 4.3%.

Traders on equity markets treat the 5% level as a notable reference point for 10-year yields — a threshold beyond which the cost of borrowing is generally expected to press on both share valuations and broader economic activity — while 30-year yields climbed further still, reaching 5.64%.

Rate decisions at the central bank shifted toward tighter monetary policy for the first time in three years, driven by consumer price data that had outpaced projections and energy costs that held at persistently high levels. Higher energy costs were among the factors that shaped the Federal Reserve's move to lift borrowing rates that September. At its September gathering, the Federal Reserve nudged its benchmark lending rate up by 25 basis points, placing the federal-funds target within a band of 3.75% to 4.00%.

The episode served as a notable test for Chair Kevin Warsh, whose limited public commentary on rate decisions and inflation during the summer had left financial markets unsettled. Warsh told those gathered after the meeting that consumer prices had remained above the Fed's target for over five years, and that the rate-setting committee's decision to act reflected its commitment to bringing inflation back under control within a more reasonable timeframe.

Investor focus on the path ahead for monetary policy took hold once the gathering concluded. Pricing in the bond futures market currently puts the probability of two additional rate increases before year-end at roughly 30%. Forecasts from Preston Caldwell, a senior economist at Morningstar, point to a pair of rate reductions in the coming year, followed by four more across 2028.