Owen Lamont, senior vice president and portfolio manager at Acadian Asset Management, has reversed course on whether U.S. stocks are in a bubble. After spending over two years arguing the market was not there yet, Lamont now says it is, with semiconductor stocks as the trigger, according to a June 2026 report by Yahoo Finance (Yahoo Finance). On July 23, 2026, CNBC referenced Lamont and his "four horsemen" framework in coverage of U.S. IPO activity and broader market conditions (CNBC).

Lamont built his case across a series of posts in Acadian's "Owenomics" series. In March 2024, he authored "No, we are not in a bubble yet," introducing his "four horsemen of the bubble apocalypse" diagnostic framework. By July 2024, a follow-up titled "Getting bubbly" referenced the "First Horseman," signaling incremental concern. A November 2024 post, "Bubble Beliefs: Prices Too High but Going Higher," described the Four Horsemen as the main symptoms of bubbles. In November 2025, "Bubble Watch, Part 1: Firms are the smart money" focused on corporate issuance and repurchase decisions as a diagnostic input.

The most recent installment, titled "We are Not in an AI Bubble" and published January 9, 2026, on Acadian Asset Management's website, presents the Four Horsemen framework in full (Acadian Asset Management). By June 2026, Yahoo Finance reported that Lamont had moved from that "no bubble" position to an explicit alarm, with semiconductor stocks identified as the catalyst for his shift.

The Four Horsemen framework is a structured diagnostic rather than a single-metric screen. Think of it as a checklist that examines multiple pillars of market health rather than relying on one number like a price-to-earnings ratio. One pillar is corporate behavior, specifically issuance (when companies sell new shares to raise capital) and repurchase activity (when companies buy back their own stock). The November 2025 post framed firms as "the smart money," meaning their financing decisions are a telltale signal of whether equity valuations have detached from fundamentals, meaning the underlying worth of the business. The November 2024 post addressed the psychological dimension: the conviction that prices are too high but will keep rising. The July 2024 post's reference to the "First Horseman" suggests a sequential or staged assessment, though the full taxonomy is distributed across multiple posts rather than enumerated in a single document.

What makes Lamont's trajectory notable is the arc itself. In March 2024, the Owenomics series opened with a declarative "no bubble" verdict. Over the subsequent 27 months, through five documented posts, the posture moved from reassurance to granular monitoring to, by mid-2026, an active alarm keyed to the semiconductor complex. The CNBC reference on July 23, 2026 places the framework into the context of U.S. IPO activity, suggesting that the corporate-finance dimension of the Four Horsemen, the issuance channel Lamont examined in his November 2025 post, is now being tested in real time.

The broader context here is what Lamont's shift means for how investors should weigh the risk. Chip stocks have been the primary vehicle for AI-driven equity re-rating, meaning the upward repricing of shares tied to artificial intelligence expectations. If Lamont's alarm is rooted in corporate-level signals like excessive issuance, insider selling, or aggressive buyback activity funded at rich valuations, rather than in price momentum alone, the diagnostic carries a different weight than a simple valuation call. The Owenomics series has consistently treated firm-level behavior as the "smart money" indicator, which means the alarm is grounded in what companies actually do with their own stock, not sentiment surveys or technical overlays.

The Yahoo Finance report did not enumerate which specific Four Horsemen conditions Lamont now sees as met versus merely approaching. CNBC's July 23 reference likewise cited the framework without detailing which horsemen are presently active. For those tracking the thesis, the primary sources remain the Owenomics posts, each of which builds one layer of the diagnostic.

The key tension is between Lamont's January 2026 declaration that AI is "not" a bubble and his June 2026 alarm. Five months separate those two statements. Whether the shift reflects new data on corporate behavior in the semiconductor supply chain, a re-rating that crossed a threshold in the framework, or a combination of both is not specified in the available reporting. What is documented is the direction of travel: from reassurance to warning, keyed to a sector that has carried the marginal bid in U.S. equities for the better part of two years.

What separates Lamont's framework from standard bubble diagnostics is its insistence on firm-level signals rather than price action alone. Most bubble screens rely on valuation multiples (like price-to-earnings ratios), sentiment indicators, or price-velocity metrics, which measure how fast prices are rising. Lamont's Four Horsemen center on what companies actually do with their own stock, the decisions corporate insiders make when they have material non-public information about their own prospects. That makes the framework a corporate-finance-first approach to a problem typically addressed through market-level data.

As of the CNBC reference on July 23, 2026, the framework is being cited in the context of live IPO activity, which directly engages the issuance channel Lamont examined in his most recent Owenomics post. Whether that activity confirms or rebuts the bubble thesis depends on the specifics of issuance volume, pricing, and post-deal performance, none of which the available sources detail. The framework provides the lens. The data will determine the verdict.