Equal Weight vs Market-Cap Weighting: Index Construction Compared

Most investors who own index funds own market-capitalisation-weighted funds without thinking deeply about what that means. The MSCI World index, the S&P 500, and the FTSE All-Share all assign each constituent a weight proportional to its total market capitalisation — the share price multiplied by the number of shares outstanding. Larger companies get larger weights, automatically.

This approach has a practical virtue: cap-weighted indices are low-cost to implement because they require almost no rebalancing (when Apple rises, its weight naturally increases, and no transaction is required to maintain it). But cap-weighting also creates a structural concentration in the largest companies that grows over time, and which — in certain periods — creates serious portfolio concentration risk.

The Concentration Problem in Cap-Weighted Indices

Consider the S&P 500 as of early 2026. The top ten holdings account for approximately 35–38% of the entire index. Apple and Microsoft alone represent roughly 12–14% combined. The top two stocks in the world's most-cited benchmark — two companies from the same country, the same sector, and correlated to the same macroeconomic drivers — represent more than one dollar in eight invested in the index.

This concentration is not unique to 2026. It reflects a structural tendency of cap-weighting: bull markets in specific sectors cause those sectors to grow as a proportion of the index, which attracts more passive inflows, which further supports prices, potentially in a reflexive cycle. At the peak of the dot-com bubble in 2000, technology stocks represented over 30% of the S&P 500. Subsequently, they fell approximately 75% from peak and dragged the index dramatically lower.

The 2020s version of this concentration is not necessarily wrong — the largest technology companies are extraordinarily profitable, cash-generative businesses with durable competitive advantages. But investors who believe they hold a "diversified" portfolio in the S&P 500 should recognise that they have significant concentration in a handful of US mega-caps.

Equal Weighting: The Alternative

In an equal-weight index, every constituent receives the same weight. In the S&P 500 Equal Weight Index, each of the 500 companies represents approximately 0.2% of the portfolio. Apple is given exactly the same weight as Dollar Tree or Hasbro.

This approach fundamentally changes the portfolio characteristics:

Reduced concentration: no single company can dominate returns. The performance of Apple no longer meaningfully determines portfolio performance.

Small-cap tilt: because smaller S&P 500 companies are overweighted relative to their market cap, the equal-weight portfolio has a structural tilt toward mid-cap and smaller large-cap companies. This introduces a size factor tilt that, over long periods, has been associated with a size premium (small companies outperforming large companies on average).

Value tilt: equal-weighting relative to cap-weighting proportionally increases exposure to companies whose market cap is lower relative to their fundamental size (revenues, assets, employees). This is a mechanical value tilt.

Contrarian rebalancing: equal-weight indices must rebalance back to equal weights periodically (typically quarterly). This rebalancing mechanically sells recent winners (whose prices have risen, making their weight exceed the equal-weight target) and buys recent losers. This is the same contrarian, buy-low-sell-high dynamic discussed in the rebalancing guide — but embedded automatically in the index methodology.

Historical Performance

The long-run evidence on S&P 500 equal-weight versus cap-weight performance is instructive:

Over the period 1990–2020, the S&P 500 Equal Weight Index outperformed the cap-weighted S&P 500 by approximately 1–2% per annum. This outperformance is broadly consistent with the small-cap and value premiums embedded in the equal-weight methodology.

However, the 2010s and particularly the 2017–2021 period saw cap-weight significantly outperform equal weight, as mega-cap technology stocks (primarily the FAANG+ group) delivered extraordinary returns. Equal-weight investors systematically underweighted the best-performing stocks. This extended underperformance led to significant ETF outflows from equal-weight products.

The pattern is cycle-dependent: equal weight tends to outperform when market leadership is broad (multiple sectors and sizes performing well, as in the 2000s recovery), and underperforms when a narrow group of mega-caps drives the market (as in the 2017–2021 growth/tech cycle).

This means that timing the switch between cap-weight and equal-weight is as difficult as timing any other factor, and investors should assess whether the long-run average advantage of equal weight is worth the extended periods of underperformance.

Higher Turnover and Cost

Equal-weight indices require regular rebalancing back to equal weights. Quarterly rebalancing of the S&P 500 Equal Weight Index generates turnover of approximately 20–25% per year, compared with near-zero for a cap-weighted fund.

This turnover creates transaction costs — brokerage commissions, bid-offer spreads, and market impact (particularly for less liquid small-cap constituents). For large AUM funds, trading into 500 positions with equal weights creates market impact that erodes the theoretical advantage of the strategy.

The Invesco S&P 500 Equal Weight UCITS ETF (SPEX) has a TER of 0.20%, but investors should examine the tracking difference — the actual gap between fund return and index return over time — which reflects total implementation cost including trading friction. For smaller, more liquid indices, the drag is modest; for broader equal-weight strategies that include less liquid names, it can be meaningful.

Liquidity at the Small-Cap End

The practical challenge of equal weighting at scale is the smaller-cap constituents. If the S&P 500 Equal Weight Index has AUM of $20 billion, it needs to hold $40 million in each of 500 stocks. For the largest stocks (Apple, Microsoft), $40 million is trivial. For smaller S&P 500 constituents with market caps of $5–15 billion, $40 million represents 0.3–0.8% of the float — a meaningful ownership stake that limits the fund's ability to trade without market impact.

For ETFs with very large AUM, this becomes a genuine constraint. The largest vehicle in this space, the US-listed Invesco S&P 500 Equal Weight ETF (RSP), holds well over $50 billion and rebalances quarterly; managing the market impact of rebalancing a fund of that size across the smaller constituents is a real operational challenge. For institutional investors or large discretionary mandates, the liquidity constraint in smaller constituents is a genuine concern.

Practical Products

Invesco S&P 500 Equal Weight UCITS ETF (SPEX): tracks the S&P 500 Equal Weight Index. Listed in London. TER 0.20%. The primary UCITS vehicle for this strategy. Adequate liquidity for retail and smaller institutional allocations.

Xtrackers S&P 500 Equal Weight UCITS ETF: alternative provider with similar methodology.

Invesco MSCI World Equal Weight UCITS ETF: extends equal weighting to developed world equities across ~1,500 stocks. Higher turnover and greater liquidity challenges at the smaller end.

For investors who want to reduce mega-cap concentration without adopting an explicit factor strategy, equal-weight indices offer a transparent, rules-based alternative. For investors already comfortable with factor investing, a multi-factor approach (adding explicit value, size, and quality tilts to a cap-weighted core) may be a more targeted method of achieving similar exposures.

The Decision Framework

The key question for each investor is: do you believe the long-run size and value premiums embedded in equal weighting justify the extended periods of cap-weight underperformance, and can you tolerate those periods without switching strategies?

If the answer is yes, an allocation to equal-weight alongside cap-weighted core positions provides diversification across index methodologies. If the answer is uncertain, a core cap-weighted position with deliberate small-cap and value tilts via separate allocations may achieve similar diversification with more controllable factor exposures.

All investments can fall as well as rise. Index strategies do not guarantee positive returns. Equal-weight strategies carry specific risks including small-cap concentration, higher turnover costs, and extended periods of underperformance versus cap-weighted equivalents. Past performance does not guarantee future returns. This guide does not constitute personal financial advice.

How Global Investments Can Help

Our team can help you assess index construction methodologies in the context of your overall portfolio and identify where equal weight, factor tilts, or active management may complement your existing core index allocations. Contact us to discuss your portfolio construction objectives.