Vanguard believes that a bubble is an instance of prices far exceeding an asset's fundamental value, to the point that no plausible future income scenario can justify the price, which ultimately corrects. Our view is informed by academic research dating from the start of this century, before the dot-com bubble burst.

Are there asset bubbles out there now? We at Vanguard have great respect for the uncertainty of the future, so the best we can say is "maybe." Some specific markets, such as U.S. housing and cryptocurrencies, seem particularly frothy. U.S. home prices rose 10.4% year-over-year in December 2020, their biggest jump since recovering from the global financial crisis.² But pandemic-era supply-and-demand dynamics, rather than speculative excess, are likely driving the rise. 

Cryptocurrencies, on the other hand, have soared more than 500% in the last year.³ It's a curious rise for an asset that is not designed to produce cash flows and whose price trajectory seems like that of large-capitalization growth stocks—the opposite of what one would expect from an asset meant to hedge against inflation and currency depreciation. Rational people can disagree over cryptocurrencies' inherent value, but such discussions today might have to include talk of bubbles.

What about U.S. stocks? The broad market may be overvalued, though not severely. Yet, forthcoming Vanguard research highlights one part of the U.S. equity market that gives us pause: growth stocks. Low-quality growth stocks especially test our "plausible future income" scenario. For some high-profile companies, valuation metrics imply that their worth will exceed the size of their industry's contribution to U.S. GDP. Conversely, our research will show that U.S. value stocks are similarly undervalued.

Low-quality growth stocks—companies with little to no operating profits—have outperformed the broad market by 5.5 percentage points per year over the last decade. Of course, there are reasons why growth stocks may be richly valued compared with the broad market. Growth stocks, by definition, are those anticipated to grow more quickly than the overall market. Their appeal is in their potential. But the more that their share prices rise, the less probable that they can justify those higher prices. A small handful of these "low-quality growth" companies may become the Next Big Thing. But many more may fade into obscurity, as occurred after the dot-com bubble.

The market for initial public offerings offers some insights (with my thanks to colleagues in Vanguard Quantitative Equity Group). Four out of five companies that offered shares on public markets for the first time in 2020 had earnings per share below zero. The percentage of such unprofitable IPOs has been nearly as high for the last several years, comparable to the numbers seen in the years leading up to the dot-com bubble. Such dynamics could create the catalyst for a hard landing for growth stocks. Of the unprofitable IPOs in 2020, more than 80% were in the technology and biotech sectors.⁴

Value stocks, by contrast, trade at prices below what company fundamentals suggest are reasonable. These steady companies seem to have been around forever and are likely to stay around. High-quality value stocks have underperformed the broad market by 15.5 percentage points per year over the last decade.

As we outlined in the  Vanguard Economic and Market Outlook for 2021: Approaching the dawn, we expect equity markets outside the United States to outperform U.S. equities and expect value stocks to outperform growth. Our December 2020 research A Tale of Two Decades for U.S. and Non-U.S. Equity: Past Is Rarely Prologue underscores our beliefs, as does our forthcoming assessment of growth and value stocks. 

Performance variation between investing styles and sectors is among the reasons Vanguard believes that investors should hold broadly diversified portfolios, as highlighted in Vanguard’s Principles for Investing Success (PDF). Those who do so benefit from a natural rebalancing that occurs over time as market segments outperform and underperform. 

However, investors with conviction in their assessment of the markets, the time horizon to be patient, and the discipline to hold firm amid volatility may benefit from an overweight allocation to value stocks commensurate with their risk tolerance. Such a tilt could help offset the lower broad-market returns we expect in the decade ahead compared with the decade past. 

We appreciate that fast-rising assets may grow richer still. But, at some point, the markets will be faced with a question related to our definition of asset bubbles: What plausible future income scenario justifies an asset’s price? We expect that valuations eventually will reflect companies’ true probability of profitability, especially in the most-stretched corners of the market. 

I’d like to thank Ian Kresnak, CFA, and my colleagues in Vanguard Quantitative Equity Group for their invaluable contributions to this commentary.