Why do a few giants move the whole index?
"The market rose 1%" — but HOW is 500 companies' worth of motion squeezed into one number? The answer (the weighting scheme) explains a modern puzzle: why the famous indices sometimes dance to the tune of a handful of stocks.
Cap-weighting: the modern default
The S&P 500 and most major indices are market-cap weighted (float-adjusted): each company's influence is proportional to its market value held by the public. A $3-trillion giant weighs hundreds of times more than the list's smallest member. Logic: the index mirrors where investors' money actually sits.
The consequence is concentration you should know as a fact of modern markets: the ten largest companies now account for well over a third of the S&P 500's total weight — 37% when this lesson was last checked against the data, in August 2026. A great day for two or three mega-caps can lift "the whole market" while the average stock naps — and vice versa. When the index and "most stocks" disagree, weighting is the translator.
The charming fossil: price-weighting
The Dow Jones Industrial Average — the oldest famous index — weights members by share price, an 1896 shortcut that never left. A $400-per-share company influences the Dow more than a far larger business trading at $80, which (as Course 1 taught you) measures... nothing meaningful. Japan's Nikkei 225 grew from the same price-based tradition (its modern formula adds adjustment factors to soften the effect). Both remain culturally beloved, professionally treated as historical curiosities — headlines quote the Dow; practitioners benchmark against cap-weighted indices.
Equal weight: the control group
An equal-weighted version of the S&P 500 exists — every member 0.2%, giants and minnows alike. Comparing it with the standard version is the cleanest live gauge of concentration: when cap-weighted races ahead of equal-weighted, mega-caps are carrying the market; when equal-weighted leads, breadth is back.
In the data
The concentration is measurable. An S&P 500 fund holds the index at its weights, so its ten largest holdings read as the index's ten heaviest members. Here they are for SPY:
Each weight is a percentage of the whole fund. Added up, the ten came to 37.0% of the fund the day this lesson was checked in August 2026: more than a third of "the market", in ten names.
Try it now
- Find a company's weight in its main index: find Apple in the table above, or your own anchor company if it made the list. Then add the ten up and set today's total beside the 37% this lesson quoted for August 2026.
- Put the standard and equal-weight S&P 500 side by side for the past year — which led? The first table is SPY, the cap-weighted fund; the second is RSP, which holds the same companies at equal weights. Compute each year's return from the adjusted close, the price with dividends folded back in: last over first, minus one. You've just measured concentration like a strategist.
- Translate one headline: "markets rallied" → "cap-weighted average of mostly a few big names rallied." Feel the precision upgrade.