Mega-IPOs from SpaceX and others could intensify already 'dangerous' concentration risk in the stock market

• Nearly 40% of the S&P 500's weight is made up of its 10 largest stocks.
• If SpaceX, OpenAI, and Anthropic go public, that weighting could jump to 50%.
• While top-heavy market concentration isn't inherently bad, experts warn it can intensify risks.

Just 10 stocks account for nearly 40% of the S&P 500's value, the highest concentration in the stock market since the Great Depression.

What's more, if SpaceX, OpenAI, and Anthropic are added later this year, that concentration could near 50%, Torsten Slok, Apollo's chief economist, said.

Today's top-heavy stock market concentration was among the risks that Richard Bookstaber, a Wall Street veteran who predicted the Great Recession, named as signs that the next financial crisis could be worse than 2008.

Morningstar strategists pointed out that market concentration isn't inherently bad, but it comes with risks.

"Even if concentration doesn't guarantee a downturn, it erodes diversification benefits and makes markets more vulnerable to sentiment reversals."

In today's market, where tech giants dominate the S&P 500, mega IPOs could intensify the risks derived from heavy concentration.

The last time the stock market was this concentrated at the top was 1932. At that time, the 10 largest stocks included names like AT&T, General Motors, DuPont, and R.J. Reynolds Tobacco.

In the age of AI, it's mostly tech and Magnificent Seven names at the top, led by Nvidia, and joined by others like Broadcom and Berkshire Hathaway.

The AI boom has fueled massive gains for the biggest tech giants, fueling the bull market and intensifying worries about market concentration.

"Over the past decade, the S&P 500, which has historically been viewed as a balanced cross-section of the U.S. economy, has slowly transformed into a tech- and AI-dominated index," RBC Wealth Management said.

The firm added that the phenomena, which they call the "Great Narrowing," should be a top concern for investors.

Increased risk of an idiosyncratic shock

When a few companies carry substantial weight in the index, it raises the risk that bad news at the top could drag the whole market lower.

Bookstaber, who called the 2008 financial crisis, warned today's level of market concentration is not only "unprecedented," but "dangerous." He explained "A shock to any one of these companies can ripple across the entire market rather than be absorbed by it."

The performance of a few stocks has an large influence on investors stock portfolios and retirement investments.

Investors face a passive concentration trap

The S&P 500 is generally considered a diversified investment in the US economy, but it has come to be dominated by tech and AI.

Investors buying the index may believe they're getting a diversified and balanced investment vehicle, but in reality, its market-cap-weighted structure means their primary exposure is to a handful of companies.

"More than $40 of every $100 invested flows into just 10 companies, creating a feedback loop where passive inflows disproportionately support the largest stocks, increasing their weights and reinforcing performance leadership regardless of fundamentals," RBC Wealth Management said,

Per Torsten Slok's projections, this $40 figure would approach $50 if SpaceX, OpenAI, and Anthropic go public and join the benchmark index.

Cullen Rogers, who manages Wedbush's EXEQ ETF, said market concentration means investors need to consider how diversified their portfolio actually is.

"Investors may think they're diversified when they're really making a structural bet," Rogers told Business Insider.

Risk associated with AI exposure rise

Not only is the S&P 500 dominated by a few mega-cap names, but these companies also fall under a common theme of AI and tech.

Investors recently saw the damage that a sudden shift in investor sentiment can do when AI disruption fears fuelled a violent sell-off in software stocks earlier this year.

"It is important for investors to remember that even when they put money into a passive instrument that tracks SPX, they are making a significant implicit investment in AI and related technologies, whether they realize it or not," Steve Sosnick, Interactive Brokers' chief strategist, said.