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How big is the stock market's America bubble?
The U.S. has grown to nearly two-thirds of global equity market value, but some analysts see danger in the 'huge bet on AI'
Author of the article:
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Today's tech dominance on Wall Street has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket.
Photo by JOHANNES EISELE/AFP/Getty Images files
United States stocksâ huge surge since the global financial crisis means they account for almost two-thirds of the
worldâs investable market
, raising concerns about whether such dominance creates too much risk for investorsâ portfolios.
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or
Wall Street
has raced ahead of international rivals over the past decade and a half, driven largely by a rally in the
tech sector,
and particularly companies linked to
artificial intelligence
, which is now worth almost as much as all the stocks in Europe combined.
But a recent pullback in tech shares has underlined the growing nervousness around soaring valuations in a market that has swallowed an ever larger share of global investorsâ allocations.
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âIf you hold a global tracker then by definition two-thirds of that is the U.S., and a lot of that is in
Silicon Valley
specifically,â said Paul Marsh, a professor of finance at London Business School who has spent the past 25 years tracking long-run investment returns.
âThat means youâre very vulnerable to this huge bet on AI.â
Consistent returns have helped the U.S. stock market balloon in size since 2010, with the countryâs share of global free-float market capitalization climbing from about 40 per cent in the aftermath of the global financial crisis to more than 64 per cent by 2025.
The U.S. has held the title of the
worldâs largest stock market
for much of the past century, having edged ahead of the United Kingdom
â
the dominant market during the 19th century
â
by the early 1900s.
By its peak in the late 1960s, the U.S. made up more than 70 per cent of the global investable market, according to the UBS Global Investment Returns Yearbook.
This high point was driven by Americaâs booming postwar economy, but also a relative lack of competition: Most of todayâs âemerging marketsâ were yet to develop significant stock markets.
But the global crash of 1973-74 hit the U.S. particularly hard. Wall Street stocks did not climb back to their late 1960s peak for more than 20 years, according to Brunel University professor of banking and finance E Philip Davis.
This decline allowed a new global leader to emerge, albeit briefly: Japan became the only country in the past century to surpass the U.S. as the worldâs largest stock market. The shift arose at the height of the late 1980s Japanese asset price bubble, which later burst.
The end of this speculative mania left foreign and domestic investors deeply skeptical about Japanâs equity markets, and its economy lay stagnant for decades. It was not until last year that the benchmark Nikkei 225 broke beyond its bubble-era peak.
âEvery now and then, finance goes off the rails and that happened in Japan. People get overenthusiastic, everybody feels rich, but then it turns out to be a house of cards,â said Richard Sylla, professor emeritus of economics at NYU Stern School of Business.
Parallels between todayâs stock market and these historical crashes are making some investors uneasy.
âThe No. 1 question I get asked at the moment is around what to do about the U.S. stock market,â said Duncan Lamont, head of strategic research at U.K. fund manager Schroder Investment Management.
However, the âstriking persistenceâ of the U.S. equity marketâs performance since 2008 makes it difficult to push against the trend, because ânaysayers have been wrong many times over,â he said.
The S&P 500 index has delivered average annual returns of about 14 per cent since 2010, outstripping all other major national benchmarks. That performance was bolstered by gains of more than 20 per cent in both 2023 and 2024, as excitement about AI pushed U.S.-listed megacap technology stocks, such as chipmaker Nvidia Corp., to record highs.
The start of 2025 has brought a rare bout of Wall Street underperformance, as relatively unloved European markets play catch-up.
U.S. dominance is also a consequence of foreign companies, particularly in the tech sector, choosing to list in New York in search of higher valuations.
Some investors argue this trend has brought many of the worldâs best companies to the U.S. and will make the market more resilient to an economic downturn.
âI can pretty much build a global portfolio just relying on U.S. markets,â said Jack Ablin, chief investment officer at private investment firm Cresset Capital.
But, for others, it is not just the outsize role of the U.S. market but also its concentration in a small number of stocks that is fraying nerves. In particular, skeptics point to the huge gains of many Silicon Valley giants, which Torsten Slok, chief economist at private capital group Apollo Global Management Inc., said had become âridiculously overvalued.â
The
Magnificent Seven group of giant technology stocks
â
Apple Inc., Alphabet Inc., Amazon.com Inc., Meta Platforms Inc., Microsoft Corp., Nvidia and Tesla Inc.
â
hold almost a third of the S&P 500âs US$51.8 trillion market value, while the indexâs cyclically adjusted price-to-earnings ratio, a measure of valuation, is approaching its highest level since the early 2000s.
âPeriods come and go where bubbles start to form. And we are in a bubble today in the U.S., and a bubble in the tech world,â said Slok.
Bullish investors argue that Big Techâs strong earnings growth and AIâs potential to spur productivity justify the lofty valuations of many of the worldâs largest companies. Bearish commentators, meanwhile, draw comparisons between todayâs market and the dotcom bubble that burst at the beginning of the millennium.
Investor confidence was shaken in January when Chinaâs DeepSeek unveiled AI advances apparently achieved using far less computing power than U.S. tech groups, casting doubt on the need for the vast capital expenditures made by Magnificent Seven companies.
This month, renewed jitters have hit the tech sector, pulling the U.S. market back slightly from all-time highs.
This is not the first time that one sector has overwhelmed Wall Street. In the 1800s, railroad companiesâ hunger for investment played a central role in the early development of the U.S. stock market. By 1900, they represented more than 60 per cent of market value.
âArtificial intelligence is the wave of the future right now, but a hundred years ago the wave of the future was rail companies. Then we had a wave of everybody buying electricity companies,â said Sternâs Sylla.
The relative decline of a dominant industry is not necessarily bad news for investors. An investor who held railroad shares since 1900 would have outperformed the broader U.S. market, according to research by Marsh in 2015. That is despite the fact that railroadsâ overall share of the market declined as companies from a plethora of other industries joined.
Even so, todayâs tech dominance
â
and U.S. dominance
â
has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket.
âThe bottom line is that if I open Page One in my finance textbook, it says that I should diversify,â said Slok.
âPeople are looking at their holdings . . . and asking a very, very fundamental question, namely: âAm I diversified?â And the answer today to that question is a very, very clear no.â
© 2025 The Financial Times Ltd
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# How big is the stock market's America bubble?
The U.S. has grown to nearly two-thirds of global equity market value, but some analysts see danger in the 'huge bet on AI'
Author of the article:

Financial Times
Stephanie Stacey and Mari Novik in London
Published Mar 04, 2025
5 minute read
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Today's tech dominance on Wall Street has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket. Photo by JOHANNES EISELE/AFP/Getty Images files
Article content
United States stocksâ huge surge since the global financial crisis means they account for almost two-thirds of the [worldâs investable market](https://financialpost.com/tag/global-markets/), raising concerns about whether such dominance creates too much risk for investorsâ portfolios.

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[Wall Street](https://financialpost.com/tag/wall-street/) has raced ahead of international rivals over the past decade and a half, driven largely by a rally in the [tech sector,](https://financialpost.com/tag/tech-sector/) and particularly companies linked to [artificial intelligence](https://financialpost.com/tag/artificial-intelligence/), which is now worth almost as much as all the stocks in Europe combined.
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But a recent pullback in tech shares has underlined the growing nervousness around soaring valuations in a market that has swallowed an ever larger share of global investorsâ allocations.
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âIf you hold a global tracker then by definition two-thirds of that is the U.S., and a lot of that is in [Silicon Valley](https://financialpost.com/tag/silicon-valley/) specifically,â said Paul Marsh, a professor of finance at London Business School who has spent the past 25 years tracking long-run investment returns.
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âThat means youâre very vulnerable to this huge bet on AI.â
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Consistent returns have helped the U.S. stock market balloon in size since 2010, with the countryâs share of global free-float market capitalization climbing from about 40 per cent in the aftermath of the global financial crisis to more than 64 per cent by 2025.
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The U.S. has held the title of the [worldâs largest stock market](https://financialpost.com/tag/u-s-economy/) for much of the past century, having edged ahead of the United Kingdom â the dominant market during the 19th century â by the early 1900s.
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By its peak in the late 1960s, the U.S. made up more than 70 per cent of the global investable market, according to the UBS Global Investment Returns Yearbook.
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This high point was driven by Americaâs booming postwar economy, but also a relative lack of competition: Most of todayâs âemerging marketsâ were yet to develop significant stock markets.
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But the global crash of 1973-74 hit the U.S. particularly hard. Wall Street stocks did not climb back to their late 1960s peak for more than 20 years, according to Brunel University professor of banking and finance E Philip Davis.
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This decline allowed a new global leader to emerge, albeit briefly: Japan became the only country in the past century to surpass the U.S. as the worldâs largest stock market. The shift arose at the height of the late 1980s Japanese asset price bubble, which later burst.
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The end of this speculative mania left foreign and domestic investors deeply skeptical about Japanâs equity markets, and its economy lay stagnant for decades. It was not until last year that the benchmark Nikkei 225 broke beyond its bubble-era peak.
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âEvery now and then, finance goes off the rails and that happened in Japan. People get overenthusiastic, everybody feels rich, but then it turns out to be a house of cards,â said Richard Sylla, professor emeritus of economics at NYU Stern School of Business.
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Parallels between todayâs stock market and these historical crashes are making some investors uneasy.
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âThe No. 1 question I get asked at the moment is around what to do about the U.S. stock market,â said Duncan Lamont, head of strategic research at U.K. fund manager Schroder Investment Management.
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However, the âstriking persistenceâ of the U.S. equity marketâs performance since 2008 makes it difficult to push against the trend, because ânaysayers have been wrong many times over,â he said.
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The S\&P 500 index has delivered average annual returns of about 14 per cent since 2010, outstripping all other major national benchmarks. That performance was bolstered by gains of more than 20 per cent in both 2023 and 2024, as excitement about AI pushed U.S.-listed megacap technology stocks, such as chipmaker Nvidia Corp., to record highs.
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The start of 2025 has brought a rare bout of Wall Street underperformance, as relatively unloved European markets play catch-up.
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U.S. dominance is also a consequence of foreign companies, particularly in the tech sector, choosing to list in New York in search of higher valuations.
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Some investors argue this trend has brought many of the worldâs best companies to the U.S. and will make the market more resilient to an economic downturn.
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âI can pretty much build a global portfolio just relying on U.S. markets,â said Jack Ablin, chief investment officer at private investment firm Cresset Capital.
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But, for others, it is not just the outsize role of the U.S. market but also its concentration in a small number of stocks that is fraying nerves. In particular, skeptics point to the huge gains of many Silicon Valley giants, which Torsten Slok, chief economist at private capital group Apollo Global Management Inc., said had become âridiculously overvalued.â
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The [Magnificent Seven group of giant technology stocks](https://financialpost.com/tag/magnificent-seven-stocks/)â Apple Inc., Alphabet Inc., Amazon.com Inc., Meta Platforms Inc., Microsoft Corp., Nvidia and Tesla Inc. â hold almost a third of the S\&P 500âs US\$51.8 trillion market value, while the indexâs cyclically adjusted price-to-earnings ratio, a measure of valuation, is approaching its highest level since the early 2000s.
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âPeriods come and go where bubbles start to form. And we are in a bubble today in the U.S., and a bubble in the tech world,â said Slok.
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Bullish investors argue that Big Techâs strong earnings growth and AIâs potential to spur productivity justify the lofty valuations of many of the worldâs largest companies. Bearish commentators, meanwhile, draw comparisons between todayâs market and the dotcom bubble that burst at the beginning of the millennium.
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Investor confidence was shaken in January when Chinaâs DeepSeek unveiled AI advances apparently achieved using far less computing power than U.S. tech groups, casting doubt on the need for the vast capital expenditures made by Magnificent Seven companies.
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This month, renewed jitters have hit the tech sector, pulling the U.S. market back slightly from all-time highs.
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This is not the first time that one sector has overwhelmed Wall Street. In the 1800s, railroad companiesâ hunger for investment played a central role in the early development of the U.S. stock market. By 1900, they represented more than 60 per cent of market value.
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âArtificial intelligence is the wave of the future right now, but a hundred years ago the wave of the future was rail companies. Then we had a wave of everybody buying electricity companies,â said Sternâs Sylla.
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The relative decline of a dominant industry is not necessarily bad news for investors. An investor who held railroad shares since 1900 would have outperformed the broader U.S. market, according to research by Marsh in 2015. That is despite the fact that railroadsâ overall share of the market declined as companies from a plethora of other industries joined.
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Even so, todayâs tech dominance â and U.S. dominance â has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket.
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âThe bottom line is that if I open Page One in my finance textbook, it says that I should diversify,â said Slok.
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âPeople are looking at their holdings . . . and asking a very, very fundamental question, namely: âAm I diversified?â And the answer today to that question is a very, very clear no.â
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*© 2025 The Financial Times Ltd*
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## How big is the stock market's America bubble?
The U.S. has grown to nearly two-thirds of global equity market value, but some analysts see danger in the 'huge bet on AI'
Author of the article:

You can save this article by registering for free [here](https://financialpost.com/register/). Or [sign-in](https://financialpost.com/sign-in/) if you have an account.

Today's tech dominance on Wall Street has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket. Photo by JOHANNES EISELE/AFP/Getty Images files
United States stocksâ huge surge since the global financial crisis means they account for almost two-thirds of the [worldâs investable market](https://financialpost.com/tag/global-markets/), raising concerns about whether such dominance creates too much risk for investorsâ portfolios.

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[Wall Street](https://financialpost.com/tag/wall-street/) has raced ahead of international rivals over the past decade and a half, driven largely by a rally in the [tech sector,](https://financialpost.com/tag/tech-sector/) and particularly companies linked to [artificial intelligence](https://financialpost.com/tag/artificial-intelligence/), which is now worth almost as much as all the stocks in Europe combined.
But a recent pullback in tech shares has underlined the growing nervousness around soaring valuations in a market that has swallowed an ever larger share of global investorsâ allocations.

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âIf you hold a global tracker then by definition two-thirds of that is the U.S., and a lot of that is in [Silicon Valley](https://financialpost.com/tag/silicon-valley/) specifically,â said Paul Marsh, a professor of finance at London Business School who has spent the past 25 years tracking long-run investment returns.
âThat means youâre very vulnerable to this huge bet on AI.â
Consistent returns have helped the U.S. stock market balloon in size since 2010, with the countryâs share of global free-float market capitalization climbing from about 40 per cent in the aftermath of the global financial crisis to more than 64 per cent by 2025.
The U.S. has held the title of the [worldâs largest stock market](https://financialpost.com/tag/u-s-economy/) for much of the past century, having edged ahead of the United Kingdom â the dominant market during the 19th century â by the early 1900s.
By its peak in the late 1960s, the U.S. made up more than 70 per cent of the global investable market, according to the UBS Global Investment Returns Yearbook.
This high point was driven by Americaâs booming postwar economy, but also a relative lack of competition: Most of todayâs âemerging marketsâ were yet to develop significant stock markets.
But the global crash of 1973-74 hit the U.S. particularly hard. Wall Street stocks did not climb back to their late 1960s peak for more than 20 years, according to Brunel University professor of banking and finance E Philip Davis.
This decline allowed a new global leader to emerge, albeit briefly: Japan became the only country in the past century to surpass the U.S. as the worldâs largest stock market. The shift arose at the height of the late 1980s Japanese asset price bubble, which later burst.
The end of this speculative mania left foreign and domestic investors deeply skeptical about Japanâs equity markets, and its economy lay stagnant for decades. It was not until last year that the benchmark Nikkei 225 broke beyond its bubble-era peak.
âEvery now and then, finance goes off the rails and that happened in Japan. People get overenthusiastic, everybody feels rich, but then it turns out to be a house of cards,â said Richard Sylla, professor emeritus of economics at NYU Stern School of Business.
Parallels between todayâs stock market and these historical crashes are making some investors uneasy.
âThe No. 1 question I get asked at the moment is around what to do about the U.S. stock market,â said Duncan Lamont, head of strategic research at U.K. fund manager Schroder Investment Management.
However, the âstriking persistenceâ of the U.S. equity marketâs performance since 2008 makes it difficult to push against the trend, because ânaysayers have been wrong many times over,â he said.
The S\&P 500 index has delivered average annual returns of about 14 per cent since 2010, outstripping all other major national benchmarks. That performance was bolstered by gains of more than 20 per cent in both 2023 and 2024, as excitement about AI pushed U.S.-listed megacap technology stocks, such as chipmaker Nvidia Corp., to record highs.
The start of 2025 has brought a rare bout of Wall Street underperformance, as relatively unloved European markets play catch-up.
U.S. dominance is also a consequence of foreign companies, particularly in the tech sector, choosing to list in New York in search of higher valuations.
Some investors argue this trend has brought many of the worldâs best companies to the U.S. and will make the market more resilient to an economic downturn.
âI can pretty much build a global portfolio just relying on U.S. markets,â said Jack Ablin, chief investment officer at private investment firm Cresset Capital.
But, for others, it is not just the outsize role of the U.S. market but also its concentration in a small number of stocks that is fraying nerves. In particular, skeptics point to the huge gains of many Silicon Valley giants, which Torsten Slok, chief economist at private capital group Apollo Global Management Inc., said had become âridiculously overvalued.â
The [Magnificent Seven group of giant technology stocks](https://financialpost.com/tag/magnificent-seven-stocks/)â Apple Inc., Alphabet Inc., Amazon.com Inc., Meta Platforms Inc., Microsoft Corp., Nvidia and Tesla Inc. â hold almost a third of the S\&P 500âs US\$51.8 trillion market value, while the indexâs cyclically adjusted price-to-earnings ratio, a measure of valuation, is approaching its highest level since the early 2000s.
âPeriods come and go where bubbles start to form. And we are in a bubble today in the U.S., and a bubble in the tech world,â said Slok.
Bullish investors argue that Big Techâs strong earnings growth and AIâs potential to spur productivity justify the lofty valuations of many of the worldâs largest companies. Bearish commentators, meanwhile, draw comparisons between todayâs market and the dotcom bubble that burst at the beginning of the millennium.
Investor confidence was shaken in January when Chinaâs DeepSeek unveiled AI advances apparently achieved using far less computing power than U.S. tech groups, casting doubt on the need for the vast capital expenditures made by Magnificent Seven companies.
This month, renewed jitters have hit the tech sector, pulling the U.S. market back slightly from all-time highs.
This is not the first time that one sector has overwhelmed Wall Street. In the 1800s, railroad companiesâ hunger for investment played a central role in the early development of the U.S. stock market. By 1900, they represented more than 60 per cent of market value.
âArtificial intelligence is the wave of the future right now, but a hundred years ago the wave of the future was rail companies. Then we had a wave of everybody buying electricity companies,â said Sternâs Sylla.
The relative decline of a dominant industry is not necessarily bad news for investors. An investor who held railroad shares since 1900 would have outperformed the broader U.S. market, according to research by Marsh in 2015. That is despite the fact that railroadsâ overall share of the market declined as companies from a plethora of other industries joined.
Even so, todayâs tech dominance â and U.S. dominance â has left many investors nervous that even a portfolio tracking a broad spread of global shares leaves them with too many eggs in one basket.
âThe bottom line is that if I open Page One in my finance textbook, it says that I should diversify,â said Slok.
âPeople are looking at their holdings . . . and asking a very, very fundamental question, namely: âAm I diversified?â And the answer today to that question is a very, very clear no.â
*© 2025 The Financial Times Ltd*
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