US Markets Show Bubble-Like Conditions, Economist Warns of AI Euphoria Risks

The Bottom Line

• US equity markets exhibit bubble-like characteristics, driven by elevated valuations, rising investor leverage, and concentrated growth in AI-related stocks.
• Key indicators like the Shiller CAPE and Buffett Indicator signal significant overvaluation, while margin debt has surged 54% year-over-year to $1.4 trillion.
• Concentration risks in the "Magnificent Seven" and the expansion of less-regulated private credit markets amplify systemic vulnerabilities.

US Markets Show Bubble-Like Conditions

As leading US equity indices continue to set successive records, debate intensifies regarding the sustainability of this valuation surge. International economist Dambisa Moyo argues that current US financial markets display a combination of factors reminiscent of periods preceding speculative bubbles. In an article published in Project Syndicate, Moyo posits that the stock market advance does not solely reflect positive economic prospects or the potential of artificial intelligence (AI), but also a significant increase in investor indebtedness and financial leverage across various market segments.

Traditional Valuation Metrics Signal Overextension

Moyo highlights warning signs in indicators traditionally used to gauge equity valuations. The Shiller CAPE (Cyclically Adjusted Price-to-Earnings) ratio, which compares stock prices to the ten-year average of inflation-adjusted earnings, currently approaches 42 points. This figure is substantially above its historical median of approximately 16. Another cited indicator is the Buffett Indicator, which measures the ratio of total stock market capitalization to US Gross Domestic Product (GDP). Considered a barometer of equity overvaluation, it currently exceeds 230%, with levels above 120% typically interpreted as a sign of an excessively valued market.

The Role of Debt in Market Ascent

Moyo contends that equity appreciation is not solely sustained by corporate performance but also by an accelerated growth in credit used to purchase financial assets. Data from the Financial Industry Regulatory Authority (FINRA) indicates that margin debt—loans taken by investors from brokerages to acquire stocks—reached $1.4 trillion, after growing 54% in just one year. The economist assesses that this volume of borrowed funds significantly amplifies the financial system's vulnerability. During market downturns, highly leveraged investors may be compelled to sell assets rapidly to cover losses, thereby intensifying corrections.

Another critical point is the expansion of the private credit market, a segment that has grown rapidly outside the traditional banking system. According to Moyo, a portion of these operations occurs with less regulatory oversight, complicating the assessment of the true risk level assumed by investors. She cites data presented by JPMorgan Chase CEO Jamie Dimon, showing that non-bank financial institutions now account for 64% of leveraged loans in the US, up from 54% in 2010. The participation of these institutions has also grown significantly in the real estate financing market. Moyo believes this migration makes it more challenging to identify where risks are concentrated should a financial shock occur. Although inflation has eased following reduced tensions in Middle East oil supply, Moyo considers it premature to rule out further US interest rate hikes. Should the cost of money remain elevated, highly indebted companies and investors could face increasing difficulties in refinancing their obligations.

AI Concentration and Systemic Vulnerability

The second factor of concern identified by Moyo relates to the strong concentration of the US stock market in the so-called "Magnificent Seven": $AAPL, $GOOGL, $AMZN, $META, $MSFT, $NVDA, and $TSLA. These companies collectively represent almost one-third of the S&P 500 index's market value. Investor expectations are that these firms will maintain high investments in data centers and AI infrastructure, with expenditures estimated at $725 billion in 2026 alone. For Moyo, however, this concentration also creates unique vulnerabilities. A significant downturn or re-evaluation in any of these dominant players could have disproportionate effects on the broader market, given their outsized weighting in major indices like the S&P 500 ($SPY).