Anthropic’s IPO Won’t Dramatically Alter the Stock Market, but It Will Increase Concentration

It’s not every day that a company prepares for an IPO acknowledging that its product might destroy humanity. As you’ve probably heard, Anthropic, maker of the Claude AI model, filed an S-1 with the Securities and Exchange Commission in preparation for listing its shares to the public, likely later this month. I recommend the analysis by my colleagues at PitchBook and Morningstar Equity Research, as well as a recent webinar.

My focus here is how Anthropic’s IPO might impact the broad stock market. What will it mean for investors in passive funds, including but not limited to those that track the Morningstar US Total Market Index? Thankfully, my colleague Alex Poukchanski has authored a helpful paper.

As Poukchanski points out, this is not the first “mega-IPO” of 2026. SpaceX SPCX listed its shares in June to great fanfare, at a valuation of roughly $1.8 trillion. Anthropic’s IPO is also expected to exceed $1.0 trillion, while several other sizable unicorns wait in the wings.

Such stratospheric valuations overstate the near-term impact of the mega-IPOs on the broad stock market. That said, SpaceX, Anthropic, and their ilk are helping to reshape market composition. They make a concentrated US stock market even more concentrated.

Anthropic’s Limited Free Float Will Mute Overall Market Impact

Morningstar Market Indexes, like those of most competitors, apply an arcane-sounding methodology called “float-adjusted market capitalization” when sizing constituents. That means a company’s stock market value is adjusted by what’s available to investors.

SpaceX, for example, came to market at a valuation of roughly $1.8 trillion and was added to the indexes within 5 days thanks to fast-track inclusion rules. But when adjusted for shares available to investors, its market capitalization was far smaller. While additional shares became available as lockup periods expired, SpaceX’s float-adjusted market capitalization stood at roughly $80 billion as of Oct. 31, 2026, making it the 136th-largest stock in the US market.

Poukchanski’s research models Anthropic’s market impact using an estimated valuation of $1.3 trillion, with roughly 5% of that floating freely. He actually doesn’t stop with Anthropic. Here’s how the largest private, venture capital-backed companies would impact the Morningstar US Total Market Index, assuming they float 5% of their value to the public upon IPO. Anthropic, for its part, would represent an index weighting of 0.08%. Even at its highest potential offering, its initial weight would climb to 0.13% of the total market index.

So, the short-term post-IPO market impact for Anthropic and other massive private companies is less dramatic than their astronomical valuations suggest, at least from the perspective of an investor holding a broad stock market index fund. Over time, float will increase. Lockup periods will expire, and early investors will sell shares. But this process will take place over time. Meanwhile, the market will weigh in on valuation.

Anthropic Will Increase Market Concentration

But there’s a second-order effect. When it comes to assigning stocks to market-capitalization segments—mega-cap, mid-cap, and so on—the Morningstar Market Indexes consider total market value, not adjusted by free float. They aim to capture the company’s full size.

SpaceX was assigned to the Morningstar US Mega Cap Index at the September 2026 quarterly index reconstitution. That had the effect of reducing the number of stocks needed to reach 70% of market value, as defined by the mega-cap segment. SpaceX displaced other mega-cap index constituents, with cascading effects across size segment indexes.

Anthropic, at $1.3 trillion, would qualify as a mega-cap stock. It would therefore push smaller companies down. As of the end of October, it took just 165 stocks to reach 70% of overall stock market value. Five years ago, that number was 246, and in 2015 it was 300.

Concentration at the top end of the market has profound consequences for indexes with fixed constituent counts. As Poukchanski shows, stocks ranked 1,001 through 3,000 represented just 3% of US market value as of the end of 2025. That’s down from 10% in the 1980s. More mega-IPOs will push that 3% number downward.

Sector concentration is another issue. Whereas SpaceX is classified as an industrials company, Anthropic will be assigned to the technology sector, as will others like OpenAI and Databricks. Technology stocks represented 36% of the Morningstar US Total Market Index as of Oct. 31. Five years ago, they were less than one-fourth.

Private Markets Booming

The fact that Anthropic could come to market at a valuation of $1.3 trillion reflects an important trend. The company has spent the past five years as a private company, dramatically scaling its business. It has grown not just by tapping into venture capital but also through funding by mutual funds and other “crossover” investors.

Not only is this a far cry from the 1990s, when companies raced to go public with little more than a domain name and statistics about “eyeballs,” it also marks a change from the early 2010s. Facebook was considered a “mega-IPO” in 2012 at a valuation of roughly $100 billion. Today it is worth $1.4 trillion on a float-adjusted basis.

With so much value-creation occurring in private markets, it’s no wonder that investors are seeking ways to invest in companies before they IPO. The SEC has proposed measures to open up private markets. As my colleagues recently covered, the results of the recent midterm elections might alter the course of those proposals.