Stock Price Bubble Dissection Requires a look at Fundamentals and Speculation

Identifying a Bubble Requires Separating Fundamentals from Speculation

Stock market bubbles are easy to identify in hindsight. The dot-com boom, the run-up to the global financial crisis, and the surge in technology stocks during the pandemic are all familiar examples. But identifying a bubble while it is forming is much more difficult. The challenge is that rising stock prices do not necessarily indicate speculation. Prices may increase because investors expect stronger earnings growth, lower future returns, or improving economic conditions. Distinguishing these fundamental explanations from a speculative bubble is one of the longstanding challenges in financial economics.

The authors propose a new approach to identifying stock market bubbles by combining traditional valuation data with surveys of investors’ expectations.Their findings also challenge the idea that bubbles matter only to financial markets. The authors show that increases in speculative stock valuations can stimulate economic activity, reduce unemployment, and influence monetary policy, even without corresponding improvements in corporate fundamentals.

Identifying a Stock Price Bubble and its Macroeconomic Implications

• Andreasen and Bro
• Review of Finance, 2026
• A version of this paper can be found here
• Want to read our summaries of academic finance papers? Check out our Academic Research Insight category

Key Academic Insights

Identifying a Bubble Requires Separating Fundamentals from Speculation

The authors develop a present-value model that decomposes stock prices into three components: expected future dividends, expected future returns, and a rational bubble. Their key innovation is incorporating survey-based expectations about dividend growth and stock prices, alongside traditional market data, to help distinguish changes in fundamentals from speculative price movements. This matters because a high price-dividend ratio alone does not establish that a bubble exists. Investors may rationally assign higher valuations to stocks when they anticipate stronger dividend growth or lower required returns. By incorporating information about what market participants actually expect, the model attempts to identify the portion of valuations that cannot be explained by these fundamental components. A rational bubble, in this framework, is a non-fundamental component of prices consistent with the model’s rational-expectations assumptions.

Bubbles Explain a Surprisingly Large Share of Stock Market Valuations

Using monthly U.S. data from January 1971 through December 2024, the authors find that the estimated bubble accounts for 61% of the variation in the S&P 500 price-dividend ratio. Expected returns account for another 42%, while expected dividend growth contributes little to explaining variation in the ratio over the full sample. These variance contributions are not additive shares of a fixed total, so they should not be interpreted as percentages summing to 100%. The findings suggest that speculative forces may play a much larger role in explaining stock market valuation fluctuations than traditional models without bubbles imply. Importantly, the authors also show that removing the estimated bubble substantially reduces the excess volatility of stock valuations and eliminates the evidence of explosive dynamics detected in the unadjusted price-dividend ratio.

The Model Identifies Several Historical Bubbles, Including a Potential AI Bubble

The estimated bubble follows several recognizable historical episodes. It represents approximately 10% of the price-dividend ratio during the early-1970s Nifty Fifty boom before declining to around 2% during the 1980s. It then rises sharply during the dot-com era, reaching 27% in January 2001. Another increase occurs around the global financial crisis, followed by a COVID-era bubble that peaks at approximately 19% in November 2021. Most notably, the estimated bubble reaches nearly 25% of the price-dividend ratio in December 2024, potentially reflecting investor enthusiasm surrounding artificial intelligence. This is a model-based historical estimate, not evidence that AI-related stocks were necessarily overvalued or that a subsequent market correction was inevitable.

Stock Market Bubbles Can Stimulate the Real Economy

The authors go beyond identifying bubbles to investigate their macroeconomic consequences. Using innovations in their estimated bubble as an instrument in a structural vector autoregression, they find that positive bubble shocks are followed by lower unemployment, higher industrial production, and increased working hours. These findings are consistent with two possible mechanisms. Higher stock valuations may increase household wealth and encourage spending, while higher market values may help companies relax financing constraints and support investment. Interestingly, inflation rises only modestly and its response is generally not statistically significant, suggesting that bubble-driven expansions may differ from conventional demand-driven economic booms.

Monetary Policy Responds to Bubble-Driven Expansions

The paper also finds that positive bubble shocks are followed by increases in monetary policy rates. The estimated policy-rate response peaks at approximately 48 basis points eight months after a positive one-standard-deviation bubble shock. Meanwhile, the spread between long-term Treasury yields and short-term interest rates declines, and corporate bond spreads narrow. These findings suggest that speculative stock market valuations can influence financial conditions and monetary policy even when underlying dividend growth and productivity do not respond significantly. The results are consistent with bubbles having broader economic consequences, although the analysis does not establish that policymakers are responding directly to the bubble itself rather than to its economic effects.

Practical Applications for Investment Advisors

Distinguish High Valuations from Speculative Bubbles

A high stock market valuation does not automatically indicate a bubble. Valuations may reflect expectations of stronger future earnings, lower discount rates, or other fundamental developments. Advisors should therefore avoid relying exclusively on conventional valuation ratios when assessing market conditions. The paper illustrates how incorporating investor expectations can provide an additional perspective on whether rising prices are supported by fundamentals or increasingly driven by speculative components.

Recognize That Bubbles Can Persist and Continue Growing
One important implication of the research is that identifying a bubble does not necessarily provide a reliable signal about when it will end. The authors’ model allows bubbles to build gradually and decline gradually, rather than requiring an immediate collapse. For advisors, this reinforces the distinction between assessing valuation risk and attempting to time the market. Even if a portion of stock prices reflects speculative dynamics, reducing exposure too early may involve substantial opportunity costs if the bubble continues expanding.

Incorporate Bubble Risk into Portfolio Construction
The finding that speculative components may explain a substantial share of valuation fluctuations has implications for portfolio risk management. Advisors may want to examine how concentrated equity exposures, particularly in popular market segments, could respond to a reversal in investor expectations. Rather than treating the estimated bubble as a direct trading signal, it may be more useful as an additional input into scenario analysis, diversification decisions, and assessments of portfolio vulnerability to large valuation adjustments.

Consider the Interaction Between Markets and the Economy
The study highlights an important feedback mechanism: rising stock valuations may support economic activity, while their reversal could remove that support. This means financial-market risks and macroeconomic risks should not always be evaluated independently. Advisors developing capital market assumptions or stress-testing portfolios may benefit from considering scenarios in which changes in equity valuations influence employment, production, credit conditions, and monetary policy. The paper provides evidence of expansionary effects from positive bubble shocks, although it does not establish that negative shocks produce equally sized effects in reverse.

How to Explain This to Clients

“Just because the stock market has risen significantly does not necessarily mean there is a bubble. Sometimes prices increase because companies are expected to generate stronger profits. Other times, investors may become increasingly optimistic and push prices higher than fundamentals alone can justify.This research develops a way to distinguish between those two possibilities.Looking at more than 50 years of U.S. stock market data, the researchers identify several historical bubbles, including the dot-com boom and a more recent period potentially associated with artificial intelligence.One particularly interesting finding is that bubbles do not affect only stock market investors. Rising stock prices can also support economic activity by making households feel wealthier and helping companies access financing.For investors, the lesson is not that we should try to predict exactly when a bubble will burst. It is that we should recognize when valuations may be increasingly dependent on investor expectations rather than fundamentals, and construct portfolios that can withstand a reversal in those expectations.”

The Most Important Chart from the Paper

Figure 3 illustrates the evolution of the estimated speculative component of U.S. stock market valuations between January 1971 and December 2024.

The figure contains two panels. The first shows the estimated bubble over time, while the second expresses that bubble as a fraction of the annual price-dividend ratio. Both panels incorporate uncertainty surrounding the model’s estimates.

The results are hypothetical results and are NOT an indicator of future results and do NOT represent returns that any investor actually attained. Indexes are unmanaged and do not reflect management or trading fees, and one cannot invest directly in an index.

Abstract

This paper develops a new approach to identifying rational stock market bubbles by combining a present-value valuation model with survey-based expectations of future dividends and stock prices. Using monthly U.S. data from 1971 through 2024, the authors identify several historical speculative episodes, including the Nifty Fifty boom, the dot-com bubble, the period surrounding the global financial crisis, the COVID-19 boom, and a recent episode potentially associated with artificial intelligence. The estimated bubble explains a substantial portion of variation in the stock market price-dividend ratio and helps account for excess volatility and explosive valuation dynamics. The authors also investigate the macroeconomic consequences of bubble shocks using a structural vector autoregression. They find that positive bubble shocks are followed by increased economic activity, lower unemployment, and higher monetary policy rates, while inflation responds only modestly. The results suggest that speculative stock market valuations can influence both financial markets and the broader economy, highlighting the importance of distinguishing fundamental price movements from bubble-driven fluctuations.

was originally published at Alpha Architect. Please read the Alpha Architect disclosures at your convenience.