When 500 Stocks Are Not as Diversified as They Look

An index is not the same thing as the market

The S&P 500 is often treated as shorthand for the US stock market. It is certainly an important benchmark, but its headline performance does not always describe what is happening across all its constituent companies.

That is because the index is weighted by float-adjusted market capitalisation. Put simply, the largest listed businesses have the greatest influence. A 5% move in a corporate giant matters far more to the index than the same move in a smaller constituent.

The basic arithmetic looks like this:

Contribution to the index = company weight x company return

If a small group represents a large share of an index, strong gains from that group can outweigh mediocre or negative performance elsewhere. The index rises, even though the experience of the average constituent may be less impressive.

This is not necessarily evidence that the index is broken. It is simply how market-cap weighting works. The important question is whether investors understand the exposure they actually own.

Why the number of holdings can be misleading

A fund containing 500 companies sounds highly diversified. By company count, it is. By economic exposure, however, the picture can be more concentrated.

Several leading holdings may share similar characteristics. They might operate in related industries, depend on comparable growth assumptions or be sensitive to the same changes in interest rates. Their share prices could also be supported by similar investor behaviour.

For a UK investor, a US index fund can introduce several overlapping exposures:

• A substantial allocation to the largest US companies
• Dependence on one national equity market
• Exposure to particular sectors and investment themes
• Currency exposure between sterling and the US dollar
• Sensitivity to the valuations placed on long-term corporate earnings

Owning many securities reduces company-specific risk, but it does not automatically remove country, sector, currency or valuation risk.

The FCA's guide to diversification explains the broader principle of spreading money across companies, sectors, countries and asset types. That is a more useful test than simply counting fund holdings.

Headline performance can hide weak breadth

Market breadth asks how widely gains or losses are distributed.

If most shares are rising together, performance is broad. If an index is being carried by a few dominant companies while many constituents lag, performance is narrow.

Neither condition tells us what will happen next. Narrow leadership can persist, while broad participation can weaken. Breadth is a diagnostic tool, not a timing signal.

One way to examine it is to compare a market-cap-weighted index with an equal-weighted version containing the same companies. Equal weighting gives each constituent a similar starting influence at scheduled rebalances, so the largest businesses cannot dominate to the same degree.

A wide performance gap between the two approaches may indicate that company size is playing a major role in headline returns. It does not prove that equal weighting is superior. Equal-weight funds bring their own trade-offs, including greater exposure to smaller businesses, more turnover and potentially higher costs.

The purpose of the comparison is to understand what has driven performance, rather than to identify an automatic replacement.

Concentration changes the shape of risk

A concentrated index can appear stable while other companies, sectors or regions are struggling. This may encourage investors to underestimate the range of outcomes inside the market.

Concentration also works in both directions. The companies that contribute most during a rise will have an outsized effect if their share prices fall. A portfolio can therefore be diversified by name but still depend heavily on a small set of earnings expectations.

Valuation matters here. When investors pay a high price for expected growth, more of the investment case rests on profits arriving in future years. Changes in bond yields, financing conditions or confidence can then have a larger effect on the present value placed on those earnings.

This does not mean expensive companies must fall, nor that apparently cheap shares must rise. Valuation is better viewed as a guide to expectations and risk than as a short-term forecast.

The relationship between economic news and share prices can also be counterintuitive. As explained in why weak jobs data can send stock markets higher, investors respond not only to the data itself but also to what it may imply for interest rates, profits and expectations.

How to test your real diversification

Investors do not need to predict the next macroeconomic event to review portfolio concentration. A practical audit can start with five questions.

1. What are the largest underlying holdings?

Look through fund labels and examine the actual companies owned. Two funds with different names may hold many of the same dominant shares.

2. Which risks are being repeated?

Separate company count from economic exposure. Several holdings may depend on the same sector, currency, region or valuation story.

3. What role does each fund serve?

A US large-cap tracker, a global equity fund and a multi-asset fund have different purposes. Problems arise when investors assume that one exposure provides diversification it was never designed to offer.

A broad global benchmark such as the MSCI ACWI IMI Index includes large, mid and small-cap companies across developed and emerging markets. That provides a useful comparison with a US-only large-cap index, although global indices can still contain meaningful US exposure.

4. Has recent performance altered the allocation?

Successful holdings naturally become a larger share of a portfolio. Rebalancing restores a chosen allocation by trimming exposures that have grown and adding to those that have shrunk.

This is a risk-control process, not a prediction that recent winners are about to reverse.

5. Could you tolerate a sharp fall in the dominant holdings?

Consider the effect on the whole portfolio if its largest underlying positions declined together. If the answer would force a sale or derail a near-term financial goal, the portfolio may be taking more concentrated risk than intended.

The FCA's five questions to ask before investing provide a useful starting point for considering risk, time horizon and whether an investment is understood.

Build resilience rather than predict reversals

Concentration is not, by itself, a reason to abandon an index. Market-cap-weighted funds can offer broad access, transparent rules and relatively simple portfolio management.

The more durable response is to know their limitations. Investors can review regional and sector allocations, consider whether other asset types have a role, control costs and keep enough accessible cash for shorter-term needs. This can reduce the risk of being forced to sell investments during a difficult period.

The key lesson is straightforward: an index may contain hundreds of companies while its results are driven by far fewer. Looking beneath the headline helps investors distinguish the appearance of diversification from the real thing.