Buying an index fund is often presented as a simple way to diversify. The basic idea is sound: instead of depending on one company, an investor owns a basket of securities at relatively low cost. But the number of names in that basket does not reveal how much of the result depends on the largest few. When leading companies gain weight in a market capitalisation weighted index, a portfolio can remain broadly invested while its return becomes more sensitive to a small group of businesses.
That is a question of exposure rather than a verdict on passive investing. The S&P 500 Top 10 Index methodology illustrates the mechanics: its constituents are among the largest S&P 500 companies and are weighted by float-adjusted market capitalisation. A broad index operates on the same weighting principle across more constituents. As share prices rise relative to the rest of the market, the leaders can take a bigger share of the index.
Many holdings can still mean concentrated outcomes
An index fund may own hundreds of stocks. An investor may therefore assume no single sector or group can dominate the outcome. Yet ownership count and economic weight are different. If the largest holdings account for a substantial fraction of the portfolio, a shock affecting those businesses can influence the fund even when most other holdings perform well.
The issue becomes more visible when leading firms share a common driver: similar revenue sources, funding conditions or market expectations. Different companies need not be identical to move together. If several businesses are priced for sustained AI spending, for example, a change in assumptions about that spending can affect them all. The right question is not merely how many tickers a fund holds, but how its major positions respond to the same event.
MSCI’s research on managing benchmark concentration frames the challenge for asset allocators. Concentration can arise naturally from the market’s composition, and departing from a benchmark introduces its own risks. Investors need to decide whether they intend to own the market as it stands or limit exposure to its largest components for a separate reason.
How market value weighting works
In a market capitalisation weighted index, the value assigned to a company reflects its eligible market value, subject to the index provider’s rules. If one company rises faster than the others, it generally commands a larger proportion of the index without an investor buying extra shares. This is an attractive feature for low-turnover investing: a fund can largely allow the market to adjust its weights.
The same mechanism means past winners influence future exposure. A rising share price may reflect stronger earnings and improve a company’s place in the index. It can also reflect high expectations that later prove difficult to meet. An index fund does not make an independent judgement about whether the price is fair. It follows the index rules and provides exposure to the market prices created by all participants.
That distinction is important when discussing the role of active investors. The BIS analysis of passive investing and securities markets examines how the growth of passive funds can affect market functioning. It does not establish that a rise in index funds alone causes every concentration episode. Prices continue to reflect the actions of many types of investor, and the relationship between fund flows and valuation is more complex than one directional story.
Concentration is also a benchmark question
If a benchmark has become more concentrated, an active manager can diversify away from its largest holdings. That decision creates tracking error: returns can differ from the benchmark for better or worse, sometimes for years. A manager who avoids a leading stock may look cautious in a downturn and underperform while that stock keeps rising. The value of the decision depends on the investor’s objectives and the price paid to implement it.
Equal weighting is another option. It gives each constituent a similar initial place rather than assigning the largest companies the greatest weight. This changes exposure but introduces more rebalancing and can increase the influence of smaller firms. A sector cap also changes the rules and may concentrate exposure somewhere else. Alternatives should be judged on costs, turnover and the kind of risk they add, not just on whether they lower the top-ten percentage.
Investors also need to look through multiple products. A person holding a broad US equity fund, a global fund and a technology fund may own the same large companies in all three. The account has several line items, but the economic exposures overlap. A portfolio review can aggregate holdings across funds, consider employer stock and examine regional as well as sector concentration.
Risk is more than a single percentage
A concentration figure is useful, but it cannot fully describe risk. The same top-ten weight can be distributed across firms with different balance sheets and industries, or across firms whose profits depend on one common market theme. Company weight, sector exposure, valuation and earnings sensitivity all matter. A large company can have strong cash flows and still be vulnerable if the expected growth embedded in its price is too optimistic.
The IMF’s Global Financial Stability Report provides context for how valuation, financing conditions and market shocks can interact. A broad investor should distinguish near-term market fluctuations from a lasting loss of portfolio resilience. Concentration may make returns more volatile under certain shocks, but it does not prove that a fall is imminent.
Liquidity also matters during stress. Index funds generally offer a convenient way to trade an interest in a diversified portfolio, while the underlying assets must still be valued and traded in the market. Investors who may need cash soon should not rely on an equity fund, concentrated or otherwise, as a substitute for a short-term reserve. The potential to recover from volatility depends partly on having time to stay invested.
What a useful portfolio check looks like
Begin with the actual holdings and benchmark, not the product name. Review the largest ten positions, the sectors they represent and the overlap with other funds. Then ask what role the allocation plays. An investor using a broad equity fund for long-term growth can accept more short-term variation than someone who expects to withdraw soon.
Next, examine costs and tax consequences before changing a portfolio. Selling to reduce one exposure can trigger taxes or transaction costs, while a more complicated strategy may add ongoing fees. Incremental rebalancing or directing new savings to underrepresented assets may address the issue more efficiently than frequent trading. The correct response varies by jurisdiction, tax status and personal circumstances.
The US Securities and Exchange Commission’s investor guide to mutual funds and ETFs helps explain that funds with different strategies and objectives carry different risks. Low cost is valuable, but it is not a substitute for understanding what a fund owns. Investors should read its holdings and stated index methodology rather than infer diversification from a label alone.
Why this does not undermine the case for simple investing
Broad index investing still offers genuine advantages: transparent rules, wide access and often low fees. Concentration is an aspect of the market an index fund reflects, not necessarily a defect unique to the fund. An investor may reasonably decide to accept that exposure as the cost of owning a representative slice of the market.
The alternative is not always superior. Active management can introduce manager selection risk, higher costs and inconsistency. More holdings or more funds can create complexity without changing underlying exposure. A thoughtful investor begins with a clear target for risk, geography and time horizon, and then chooses funds that collectively meet it.
Portfolio concentration deserves attention because the route from a successful company to a large index weight is automatic. Investors do not need to forecast the next market leader to manage that fact. They need to know how much of their wealth depends on today’s leaders, whether their other assets provide a genuine counterbalance and how they would respond if the leaders stopped setting the pace.
Global exposure may repeat the same bet
A global equity fund can look like a separate source of diversification beside a US equity fund, yet global market capitalisation indices often have substantial US exposure. If the largest US companies also appear prominently in the global fund, adding the second product may increase rather than dilute those particular holdings. A holdings-level look-through is more informative than the number of country labels on a product page.
Currency adds another layer. A global investment may be denominated in a foreign currency, but the economic exposure depends on where companies earn revenue, incur costs and face competition. A large company listed in one country may generate income across many countries. Simple labels can therefore obscure both overlap and diversification. The objective is to understand the main drivers of portfolio results, not to create a false impression of precision.
Investors who build portfolios with target regional or sector weights should decide how much drift they are prepared to tolerate. A periodic review can check whether market moves have altered the intended balance. Rebalancing to the original allocation is one way to control concentration, but it may incur taxes and trading costs. These costs should be weighed against the risk being reduced.
The relationship between price and fundamentals
A stock can gain weight in an index because investors expect its earnings to grow. Sometimes that expectation is justified by strong business performance; sometimes the price rises faster than the underlying profits. A fund tracking the index captures both possibilities. It is not inherently a bet on an irrational market, but neither does the benchmark guarantee fair valuation.
Investors can therefore use concentration as a prompt for a broader question about assumptions. What share of expected portfolio returns depends on unusually high margins or rapid growth persisting? How much would valuations change if interest rates or growth forecasts shifted? Precise predictions are unlikely, but comparing a few plausible scenarios can prevent one optimistic story from dominating a long-term plan.
It is also possible for a concentrated benchmark to be more resilient than a more evenly spread one in a particular downturn if its largest companies have stronger balance sheets. Concentration is a structure, not a forecast. Its significance depends on the underlying firms and the shock the portfolio faces.
What institutions can do differently
Pension funds and other large investors often measure exposures against policy benchmarks. For them, a concentrated index raises a governance question: should risk limits be set against absolute exposure to an issuer, a sector or a common business driver? A fund can be close to its benchmark and still hold a concentration that matters to its beneficiaries.
The solution need not be frequent tactical trading. Institutions can make their tolerance for issuer and sector concentration explicit, test overlapping mandates and explain why any benchmark departure is acceptable. A benchmark remains useful for accountability, but it should not replace an independent view of total portfolio risk. The same principle applies at smaller scale to individuals who use a model portfolio or default retirement fund.
Questions readers may ask
Does index concentration mean I should sell my fund
Not necessarily. The answer depends on your overall portfolio, time horizon, tolerance for losses, costs and other exposures. The first step is to identify what you actually own.
Is an equal weight fund automatically safer
No. It reduces reliance on the largest companies, but adds different exposures and usually requires more rebalancing. Its returns and risks can diverge substantially from a standard market weighted benchmark.
Can several funds hold the same companies
Yes. Broad, regional and sector funds frequently overlap. Consolidating their largest holdings gives a more useful picture than counting how many funds appear in an account.
This article provides general information and is not personal investment advice.
Sources
S&P 500 Top 10 Index methodology
MSCI’s research on managing benchmark concentration
BIS analysis of passive investing and securities markets
IMF’s Global Financial Stability Report
US Securities and Exchange Commission’s investor guide to mutual funds and ETFs