Every investor has heard of the S&P 500. It’s the benchmark that financial advisors quote, the number that flashes across news tickers, and the index that underlies trillions of dollars in retirement accounts. But most people assume it’s simply a list of the 500 biggest American companies. It’s not — and that distinction matters more to your portfolio than you might think.
It’s Not Just About Size
The S&P 500 is maintained by S&P Dow Jones Indices, and entry is governed by a specific set of rules — plus the judgment of a largely anonymous selection committee that meets regularly to review candidates.
To even be considered, a company must clear several hurdles. It must be a U.S.-based company with a market capitalization of at least $20.5 billion (as of recent thresholds). Its shares must be sufficiently liquid, meaning they trade in high enough volume to allow large funds to buy and sell without disrupting the price. At least 50% of the company’s shares must be available to the public — what’s known as the “float.”
Then comes the requirement that surprises many people: the company must have reported positive as-reported earnings for the most recent quarter, and its cumulative earnings over the prior four quarters must also be positive. That rule famously kept Tesla out of the index for years despite its enormous size and cultural cachet. The electric vehicle maker finally joined in December 2020, triggering one of the largest index reshuffles in history.
Once a candidate clears those quantitative hurdles, the selection committee makes the final call. The committee’s identities are kept confidential to prevent front-running — the practice of traders buying a stock before an anticipated announcement to profit from the resulting price surge.
Why Getting In (or Out) Sends Stocks Soaring — or Sinking
Index inclusion isn’t just a symbolic honor. It has immediate, measurable financial consequences.
When a stock is added to the S&P 500, every fund that tracks the index must buy it — and there is an enormous amount of money doing exactly that. As of 2024, approximately $7.5 trillion in assets are directly indexed to the S&P 500, with trillions more benchmarked against it. That mandatory buying surge can send a newly included stock up sharply in a matter of days.
When Tesla was added, its stock jumped more than 70% in the weeks surrounding the announcement, partly driven by index funds rushing to establish their required positions. On the flip side, companies removed from the index often see their stock drop as funds are forced to sell their holdings.
This phenomenon — sometimes called the “index effect” — is a direct consequence of the explosive growth of passive investing. Index funds and ETFs now account for roughly 45% of all U.S. fund assets, up from just 15% in 2005, according to Morningstar data. The more money that passively tracks an index, the more powerful inclusion or exclusion becomes.
What This Means for Everyday Investors
For the average investor holding an S&P 500 index fund, the mechanics of inclusion work largely in your favor. The index’s rules are designed to ensure quality — profitable companies with genuine market presence — which means your fund automatically inherits a degree of built-in filtering that pure market-cap weighting alone wouldn’t provide.
But there are quirks worth understanding. Because the committee has discretion, the process isn’t perfectly predictable. Some large companies have waited years for inclusion while smaller ones got in sooner. Palantir Technologies and Dell Technologies, for instance, both had sizable market caps for considerable time before finally being added. Meanwhile, a company can be removed not just for poor performance but for corporate restructuring, mergers, or falling below thresholds.
The broader takeaway is that the S&P 500 is less of a passive snapshot of the American economy and more of a curated, rules-based portfolio with active human oversight baked in. That’s not necessarily a bad thing — the index’s long-term track record is hard to argue with — but it’s a reminder that even the most “passive” investment strategies have structure and judgment built into their foundations.
As passive investing continues to grow, the gatekeeping role of indexes like the S&P 500 will only become more consequential — making it worth understanding exactly who’s holding the keys.