What an Index Actually Is

'The market was up today' almost always means the S&P 500 was up. But the S&P 500 isn't the market, it isn't automatic, and it isn't even fully rules-based. A committee picks it.

Last verified 2026-07-16

An index is a rulebook, and someone wrote it

An index is a defined list of holdings with a defined method for weighting them. It is a measuring instrument, not a market — and someone designed it, so their choices show up in the reading.

The S&P 500 is not simply 'the 500 biggest US companies'. A committee at S&P Dow Jones Indices selects members against published criteria — size, liquidity, US domicile, profitability — and applies judgement within them. Qualifying on paper does not get you added; Tesla was passed over more than once before it went in.

That makes the most-quoted benchmark in the world partly discretionary. A total US stock market index, holding essentially everything investable, is the more mechanical instrument — and holds thousands of companies rather than 500.

Weighted by size, adjusted for what you can actually buy

Once the members are chosen, something has to decide how much of each the index holds. The S&P 500 weights by market capitalization: a company's share of the index equals its share of the group's total value. Worth twice as much, twice the weight.

It also float-adjusts. Only shares actually available to the public count — stakes held by founders, governments or parent companies are stripped out — because a fund tracking the index could never buy them. The adjustment keeps the index replicable.

Constituents and weights are reviewed quarterly, but weights do not need rebalancing to drift: as prices move, a cap-weighted index automatically holds more of whatever has risen. Not a flaw, but it does mean the index quietly concentrates over time without anyone deciding to.

Why the biggest names move the number

Weighting by size has a consequence people find counter-intuitive: percentage moves at small members barely register, while modest moves at the giants dominate.

What actually moves a cap-weighted index is dollars of value created, not percentages. A $3 trillion company rising 1% adds $30 billion. A $10 billion company rising 40% adds $4 billion. The spectacular move is worth less than a seventh of the boring one.

So '500 companies' overstates how diversified the S&P 500 is. A large share of its movement comes from its largest handful of members, and that concentration has grown substantially in recent years. It is still far more diversified than owning one stock — just not the 1-in-500 spread the name suggests.