Equal-Weight and Alternative Index Weighting

The S&P 500 is famous, but it is only one methodology wearing 500 famous names. Its weighting rule — bigger companies get bigger weights, in direct proportion to market value — is a choice, not a law of nature, and it has a specific consequence: the seven or eight largest companies can make up close to a third of the entire index's weight, so the index's return is disproportionately the return of its biggest names. Alternative weighting schemes exist precisely to change that relationship.

Market-cap weighting and its natural drift

In a market-cap-weighted index, a stock's weight is its share price times shares outstanding, divided by the sum of that figure across every constituent. The key operational feature is that this weighting is self-rebalancing: if a stock's price doubles, its market cap doubles, and its index weight automatically rises — the index never has to trade a share to reflect that, because the weight recalculates itself continuously as prices move. This is cheap to run (near-zero turnover from price moves alone) but means winners keep getting relatively bigger, concentrating the index into whatever has already gone up.

Equal weighting

An equal-weighted index gives every constituent the same weight — 1/500th each, for a 500-stock index, at each rebalance. Because prices then drift apart between rebalances, the index must be periodically rebalanced (commonly quarterly), selling names that outperformed back down to the average weight and buying names that underperformed back up to it. This creates real turnover and trading cost, but it has a clear byproduct: an equal-weight index is mechanically tilted toward smaller companies relative to its cap-weighted cousin, since the smallest constituents get exactly the same weight as the largest, and it systematically sells recent winners and buys recent losers, a built-in contrarian rebalancing discipline.

Other alternative schemes

Fundamental weighting replaces market cap with an accounting measure — sales, book value, dividends, cash flow — as the basis for weight, on the theory that price can be temporarily wrong but revenue is a more stable anchor, avoiding the "buy more of what just got expensive" mechanics of cap weighting. Float-adjusted weighting, which most major cap-weighted indices actually use today, counts only the shares genuinely available to trade, excluding closely-held insider or government stakes, so index demand matches what the market can actually absorb. Factor-tilted or "smart beta" weighting adjusts weights toward a targeted characteristic — low volatility, high quality, high momentum — deliberately departing from both cap and equal weighting to chase a specific risk premium.

equal-weighted

A worked example

Consider a 5-stock index: A ($800bn market cap), B ($400bn), C ($200bn), D ($100bn), E ($50bn); total market cap $1,550bn. Cap-weighted, A's weight is $800/1{,}550 = 51.6\%$ and E's is $50/1{,}550 = 3.2\%$ — A alone outweighs B, C, D and E combined. Equal-weighted, every stock gets exactly 20%, so a $10,000 portfolio holds $2,000 of each, regardless of size.

Now suppose over the next quarter A rises 30% and E falls 20%, with B, C, D flat. In the cap-weighted index, no trade is needed — A's weight simply rises to about $1{,}040/1{,}790 \approx 58\%$ automatically. In the equal-weighted index, the $2,000 invested in A grew to $2,600 and the $2,000 in E fell to $1,600; at the next quarterly rebalance, the fund sells A back down and buys E back up until each holds roughly $(2{,}600+1{,}600+2{,}000\times3)/5 = 2{,}040$, i.e. about $2,040 again — mechanically selling the winner and buying the loser, which is the source of both equal weight's higher turnover and its documented small-cap and contrarian tilts over long periods.

• Higher turnover means higher cost and tax drag. Equal-weight funds routinely turn over 20–30% of the portfolio a year from rebalancing alone, versus a low single-digit percentage for cap weighting.
• "Smart beta" is a marketing umbrella, not one strategy. Always check what the alternative weight is actually tied to — fundamentals, volatility, momentum — since each produces a different portfolio and a different risk profile.
• Alternative weighting changes index-inclusion mechanics too. A stock added to an equal-weight index gets the same-size buy as every incumbent, unlike a cap-weighted addition, whose buy size depends entirely on its market value.