Cap-weighted or equal-weighted: the S&P 500 isn't as diversified as its name suggests
4 min read by Opthest
“I own the S&P 500, I’m diversified across 500 companies.” It’s the most common sentence in investing, and it hides a choice most people never made on purpose: how much of each of those 500 companies you actually own. By mid-2026, reporting pegged the Magnificent Seven’s share of the S&P 500 at roughly 30-35% of the index’s market value — seven names out of five hundred carrying close to a third of the basket. At the same time, equal-weight funds — which hold the same 500 companies but size each position differently — were posting a noticeably different year. Same index, same constituents, two very different portfolios. The difference is entirely in the weighting rule.
Isn’t the S&P 500 already diversified by definition?
It’s diversified in names, not necessarily in impact. The standard S&P 500 is capitalization-weighted: each company’s slice of the fund is proportional to its total market value, so the biggest companies drive most of the index’s return and risk. That’s not a flaw — it mirrors how much capital the market has actually allocated to each firm — but it means “buying the index” is not a neutral, hands-off decision. It’s a bet that today’s largest companies keep deserving their size. Reporting this year put the S&P 500’s top-ten concentration above levels last seen decades ago, with the Magnificent Seven alone said to represent close to a third of the index.
What does “equal-weighted” actually change?
An equal-weight fund like the well-known RSP holds the identical 500 constituents, but starts every one of them at about the same small slice — roughly 0.2% each — and rebalances back to that split every quarter. The practical effect: reporting in June 2026 put the Magnificent Seven’s weight inside that fund at only around 1.4%, versus close to a third in the standard cap-weighted version. It’s the same 500 companies, wearing a completely different portfolio.
So which one actually wins?
Neither, consistently — and that’s the point. Coverage in June 2026 showed the equal-weight version outrunning the cap-weighted S&P 500 by a couple of percentage points over the year to date, as strength broadened into small- and mid-cap names beyond the mega-caps. But zoom out and the picture flips hard: over the stretch from January 2022 through mid-2026, and again over five years, the cap-weighted index outran its equal-weight sibling by a wide margin — and in 2023 specifically, when the Magnificent Seven produced nearly all of the market’s gain, equal-weight lagged cap-weight by more than ten points. Cap-weighting wins when a narrow group of giants leads; equal-weighting wins when leadership broadens. Nobody rings a bell when the regime turns.
Is this really just an S&P 500 story?
No — it’s a general lesson about every index and every diversified portfolio you’ll ever build. A weighting rule is never “neutral”: cap-weighting is a bet on momentum and scale, equal-weighting is a forced tilt toward smaller, less crowded names, and neither is the market in some pure, assumption-free sense. Both are methodologies, and methodologies have consequences that show up exactly during regime shifts, not during calm stretches. The same look-through discipline from ETF overlap applies here: it’s not enough to know you own “the S&P 500” — you need to know which weighting scheme decided how much of each company that actually means.
How do you know how concentrated your own portfolio is?
The same question applies one level up, to a multi-asset portfolio you build yourself: do you size positions by market value, split them equally, or let an explicit objective decide? A mean-variance optimizer and a risk-parity approach are, in effect, two more weighting rules — one targets the best trade-off between expected return and variance given the covariance between assets, the other targets equal risk contribution per position rather than equal dollar size, which is itself a distinct answer from plain equal-weighting. There’s no universally “right” scheme — only the one that matches what you actually intend to bet on, checked against how concentrated the result really is, not how many line items sit in the account.
What this is NOT
This isn’t a call to switch from a cap-weighted fund to an equal-weight one, or the reverse, and it isn’t a forecast of which will win from here. The numbers above are historical performance and concentration figures, reported at specific points in 2026 — they describe a mechanism, not a recommendation, and past regimes don’t guarantee the next one. Which weighting exposure you’re comfortable holding, and how concentrated you let any single portfolio become, stays your decision.
An index isn’t “the market” handed to you unfiltered — it’s a weighting rule wearing the market’s clothes. Know the rule before you assume you’re diversified.