Market-cap weighting vs equal weight

Every index has to answer a question: how much of each stock? The standard answer — market-cap weighting — holds companies in proportion to their total value, so giants dominate. The alternative — equal weighting — gives every company the same slice. The same 500 stocks produce noticeably different portfolios, and the debate about which is 'right' teaches more about how markets work than most books.

What market-cap weighting really is

Cap weighting owns the market as it actually exists: if Apple is 7% of the total value of the S&P 500, it's 7% of your fund. This has a beautiful property — it's self-maintaining. When a stock doubles, its weight doubles automatically; no trading required. That's why cap-weighted funds have near-zero turnover, near-zero internal trading costs, and expense ratios of 0.03%. It's also, by definition, the average investor's portfolio — you can't collectively beat it, only pay more or less to hold it.

What equal weight changes

An equal-weight S&P 500 fund holds 0.2% of each company, rebalancing quarterly back to equal. Compared to cap weight, this systematically tilts toward smaller companies (the 400th-largest gets the same weight as Apple) and adds a rebalancing rhythm that trims recent winners to buy recent laggards. In factor terms, equal weight is a size tilt plus a mild anti-momentum trade — not magic, just different exposures with higher turnover, higher fees (typically 0.20% vs 0.03%), and worse tax efficiency.

The scoreboard is honest: it trades places

Equal weight beat cap weight handily from 2000–2010 (it dodged the tech-bubble concentration, then rode the small-cap recovery). Cap weight crushed equal weight through most of 2015–2024 as megacap tech compounded. Over very long horizons, equal weight has shown a modest edge before costs — consistent with the small-cap tilt — but after its higher fees, turnover, and taxes, the advantage shrinks toward noise. Anyone selling either as obviously superior is selling a rearview mirror.

How to choose (or not)

• Default to cap weight: cheapest, most tax-efficient, self-rebalancing, and definitionally market-matching.
• If you want equal weight's actual exposures (smaller companies), consider getting them directly with a cheap small-cap or extended-market fund instead.
• Hold equal-weight funds in tax-advantaged accounts — quarterly rebalancing generates distributions in taxable ones.
• Compare expense ratios: paying 0.20% vs 0.03% needs a reason you can say out loud.
• Whatever you pick, keep it for a decade. Alternating between them after each one's winning streak is the only guaranteed way to lose with both.

The bottom line

Cap weighting is the default because it's the market itself — free to maintain, tax-gentle, and impossible to collectively beat. Equal weight is a legitimate, higher-cost tilt toward smaller companies that shines in some decades and lags in others. Neither is a free lunch; they're different seats in the same theater. Take the cheap seat unless you have a real, articulable reason — and if your reason is 'concentration is scary,' diversify outward with international and small-cap funds instead of sideways.

The concentration numbers driving the debate

The reason this once-academic question became a dinner-table one is visible in a single statistic: by 2025 the ten largest companies made up roughly 35% of the S&P 500's value — the highest concentration in more than fifty years, up from about 18% a decade earlier. A cap-weighted investor's fortunes now ride heavily on a handful of mega-cap technology names; the 'diversified' index behaves, at the top, like a focused bet on AI-era giants. Equal weight mechanically dissolves that: every company gets 0.2%, the top ten shrink from 35% of the fund to 2%, and average company size drops sharply, giving the fund a persistent tilt toward smaller and cheaper stocks. Whether that tilt is a feature or a bug depends entirely on the decade you ask — it is why equal weight outran cap weight badly in 2000-2010 (when the dot-com giants deflated) and trailed badly in 2015-2024 (when the giants did the winning).

Two practical notes if you do add an equal-weight sleeve. First, the fee gap is permanent and certain while the performance edge is cyclical and uncertain — RSP's 0.20% versus VOO's 0.03% means you start every year 0.17% behind. Second, the quarterly rebalancing that defines the strategy generates more taxable distributions than a cap-weighted fund, so the sleeve belongs in an IRA or 401k when possible. Sized at 10-20% of your stock allocation and left alone for decades, it is a defensible diversification of the concentration bet; swapped in and out based on which style just had a good run, it becomes one more way to buy high and sell low.