The world's default equity benchmark — and, increasingly, a concentrated position in a handful of technology companies wearing the costume of a diversified index.
More money is benchmarked to the S&P 500 than to any other index in the world. It is the number that means "the market" in American financial conversation, the default option in most retirement plans, and the yardstick against which active managers are measured and usually found wanting.
It is also widely misunderstood in two specific ways: people assume it holds the 500 largest US companies, and they assume that holding 500 companies means it is diversified. Neither is quite true.
Standard & Poor's had been publishing stock indexes since the 1920s, but the modern S&P 500 dates to 1957, when computing power finally made it practical to calculate a 500-stock, capitalization-weighted average continuously through the trading day rather than once at the close. That technical constraint shaped the design more than any theory about the right number of companies — 500 was close to what the computing of the day could sustain in real time.
Its cultural position came later, and came from index funds. Once it was possible to buy the index cheaply, the S&P 500 stopped being a measurement and became a destination for capital — which changed the consequences of being added to it.
Selection is by committee, not formula. A company must be US-domiciled, listed on an eligible exchange, meet minimum float and liquidity thresholds, and have posted positive earnings in the most recent quarter and cumulatively over the prior four. But meeting the criteria makes a company eligible, not a member. A committee at S&P Dow Jones Indices decides.
This is why large, well-known US companies can trade for years without inclusion, and why membership announcements are genuine news rather than an arithmetic result. Critics call the discretion opaque. Defenders point out that a purely mechanical rule would force the index to buy at the top of every bubble and sell at the bottom of every crash.
Weighting is by float-adjusted market capitalization. Each company's influence is its market value adjusted to exclude shares not available to public investors — founder stakes, government holdings, cross-shareholdings. A company with a large market value but a small public float carries less weight than its headline size suggests.
This is the defining characteristic of the modern index and the thing worth understanding before anything else.
Ten companies now account for over 36% of the index, against roughly 23% at the turn of the century. Cap weighting is doing exactly what it was designed to do — the biggest companies got much bigger — but the practical consequence is that "diversified across 500 companies" describes the constituent list rather than the risk being taken.
The standard check is the equal-weighted version of the same index. When the cap-weighted S&P 500 rises sharply and its equal-weighted twin does not, the move came from the top handful of names rather than from the market broadly. That divergence has been unusually wide in recent years.
Because trillions of dollars track the index mechanically, addition creates forced buying and removal creates forced selling. The effect is real, well documented, and largely front-run: the price move typically occurs on the announcement rather than on the effective date, because traders anticipate the index funds.
This creates a genuine circularity worth noticing. A company becomes large enough to be added; addition brings mandatory buying; buying makes it larger. Cap-weighted indexing mechanically allocates more capital to whatever has already appreciated most.
Across sectors and companies, nominally yes. In terms of risk, less than the name implies: over a third of the index sits in ten companies, most of them in related technology businesses exposed to similar demand drivers.
There is no fixed schedule. Changes happen as needed — typically when a constituent is acquired, falls below eligibility thresholds, or the committee determines the index better represents the market with a different company.
Several companies have two listed share classes included separately. The company count is 500; the security count is higher.