When you buy an S&P 500 ETF, you are not buying 500 equally sized companies. You are buying a portfolio in which the largest handful of firms can dominate the return, and the smallest barely move the needle. The same logic applies to the Nasdaq 100 and the DAX, though each index uses its own rules.
Index providers publish those rules, and the details matter more than most retail investors assume. Selection criteria determine which companies qualify, while weighting determines how much each one counts, and rebalancing determines when the mix changes. Read the methodology, and you understand what risk you are actually taking.
Selection rules decide who gets in
An index is a rulebook, not a portfolio manager’s opinion, because the provider writes down criteria in advance and applies them on a schedule.
For the S&P 500, the starting filter is US domicile, a minimum market capitalization, a public float requirement and a track record of positive earnings. A committee at S&P Dow Jones Indices makes the final call. That discretion is deliberate: the committee can reject a company that passes the numeric screens or admit one that sits near a boundary. The index targets roughly 500 constituents, but the exact count drifts as companies are added and removed.
The Nasdaq 100 is narrower by design, as it holds the largest non-financial companies listed on the Nasdaq exchange. Financial firms are excluded, which is why you will not find major US banks in it. The exchange listing requirement is the defining feature, not the sector mix you might assume from the index’s reputation.
The DAX covers Germany’s largest listed companies by market capitalization and order book turnover. Deutsche Boerse sets the rules, and the index has reportedly expanded its member count over time. Eligibility depends on listing venue, legal form and liquidity, not on a committee picking favorites.
One consequence is easy to miss: an index can be concentrated in a single country, a single exchange or a single sector without any of that appearing in the name. While the Nasdaq 100 sounds like a broad technology gauge, it is in fact a listing-based index that happens to carry heavy technology weight.
Weighting methods turn selection into exposure
Once the members are chosen, the provider decides how much each one counts, and three methods dominate.
Market-cap weighting gives each company a share proportional to its free-float market value. A company worth ten times more than another gets roughly ten times the weight. The S&P 500, the Nasdaq 100 and the DAX all use variations of this approach, with adjustments for shares that are not freely tradable.
Price weighting gives each stock a weight based on its share price, and the Dow Jones Industrial Average still works this way. A high-priced stock moves the index more than a low-priced one, regardless of company size. That is a quirk of history rather than a statement about economic importance.
Equal weighting assigns the same weight to every member, so providers must then rebalance regularly to restore equal weights, because winners naturally grow larger. Equal-weight versions of major indices exist, and they behave differently from their cap-weighted parents.
Cap weighting has a self-reinforcing property: as a stock rises, its index weight grows, so index funds must buy more of it, and as it falls, funds sell. This is not a judgment about valuation. It is arithmetic, and it means a cap-weighted index automatically drifts toward whatever has already performed well.
Concentration risk is the price of cap weighting
Here is the part that catches people off guard: a cap-weighted index with hundreds of members can still behave like a small basket of stocks.
If the top ten holdings make up a large share of the index, then the other hundreds of companies contribute little to the daily move. You own them, but they barely register. When the leaders stumble, the index stumbles with them, and diversification offers less protection than the member count suggests.
This is not a flaw unique to any single index. It is a structural feature of cap weighting in a market where a few firms grow very large. The counter-argument is straightforward: those large companies earned their weight through actual market value, and fighting that trend means deviating from the benchmark you are trying to track.
For a long-term investor, the practical question is whether the concentration matches your tolerance. If a handful of names drive most of your return, your portfolio is less diversified than the headline number implies. Checking the top holdings and their combined weight takes a few minutes and tells you more than the index name does.
Rebalancing adds another layer, because regular reconstitution can force index funds to sell winners and buy laggards, or the reverse, depending on the rules. Additions and deletions also create predictable trading around the effective date, which matters to anyone watching short-term price behavior.
What this means for traders on a weeks-to-months horizon
If you trade index products over weeks or a few months, construction details shape the setups you see.
Concentration cuts both ways. A narrow leadership group can push an index higher with little participation from the rest of the market. That produces steady-looking charts that reverse quickly when the leaders turn. Breadth measures, which compare advancing stocks to declining ones, can warn you when the index level and the underlying participation disagree.
Rebalance dates are scheduled events. Providers announce changes in advance, and funds tracking the index must adjust positions around the effective date. That can create volume and price pressure in the affected stocks, independent of any news about the companies themselves. Knowing the calendar helps you separate mechanical flows from genuine shifts in demand.
Weighting method also affects how a product responds to a sector move. A cap-weighted Nasdaq 100 tracker and an equal-weight version of a similar universe will not react the same way to a rally in the largest names. If you trade the wrong vehicle for your thesis, the position can underperform even when the thesis is right.
Finally, check which index your instrument actually tracks, because two ETFs with similar names may follow different versions of the same benchmark, with different weighting or eligibility rules. The ticker alone does not tell you.
What this means for investors on a six-month-plus horizon
Over longer holding periods, construction rules influence the return you collect and the risk you carry.
Cap weighting means you are always holding the market’s current winners at their current size. Historically, that has captured broad economic growth without requiring you to pick stocks. It also means you inherit whatever concentration the market develops. You do not get to opt out of that while staying in a cap-weighted index fund.
If concentration concerns you, the alternatives have costs. Equal-weight funds rebalance more often, which can mean higher turnover and different tax outcomes. Fundamental-weighted or factor-based products deviate from the benchmark, so they can lag it for extended periods. There is no free fix, only a different set of trade-offs.
Country and currency exposure deserve the same scrutiny. The DAX gives you German large caps, but many of those companies earn revenue globally, so the index is not a pure bet on the German economy. The S&P 500 and Nasdaq 100 are US-listed, but their constituents also sell worldwide. Index labels describe listing and domicile, not revenue sources.
For UK-based investors, fund domicile and reporting status may affect how gains and dividends are taxed. That is a separate question from index construction, but it belongs in the same decision. A well-built index tracked through an unsuitable wrapper still creates problems.
A practical routine: look up the top ten holdings and their combined weight, read the selection criteria, and check the rebalance schedule. Do this once when you buy and again when the weights shift materially. It takes less time than reading a fund’s marketing page.
Conclusion: read the rulebook before you buy the ticker
The S&P 500, Nasdaq 100 and DAX are not interchangeable snapshots of the market. Each one selects members by its own criteria and weights them by market capitalization with specific adjustments. The Nasdaq 100 restricts itself to one exchange and excludes financials. The DAX focuses on German listings. The S&P 500 relies partly on committee judgment.
Those choices produce different concentration profiles, different sector tilts and different reactions to the same market event. None of this tells you which index will perform best. It tells you what you own, which is the part you can actually control.
Traders should watch rebalance dates and breadth. Investors should check top-holding weights and decide whether the concentration fits their plan. Both groups should confirm which index version their fund tracks before assuming the name means what it seems to mean.
- The major stock exchanges: how NYSE, NASDAQ, Xetra and others shape your trades
- How a Stock Split Works and What It Changes
- What is leverage and how does it work in stock trading?
Frequently Asked Questions
What is the difference between the S&P 500 and the Nasdaq 100?
The S&P 500 draws from US companies across major exchanges and includes financial firms, while the Nasdaq 100 only includes non-financial companies listed on the Nasdaq exchange. Both are market-cap weighted, but the Nasdaq 100 is narrower by listing venue and carries a heavier technology tilt as a result.
Why does the DAX have fewer members than the S&P 500?
The DAX tracks the largest companies listed in Germany, so the eligible universe is much smaller than the US market. Deutsche Boerse sets the criteria around market capitalization, liquidity and listing requirements, and the member count has been adjusted over time.
What is concentration risk in an index fund?
Concentration risk means a small number of holdings drive most of the index's movement, even when the index lists hundreds of companies. With market-cap weighting, the largest firms can carry weights that make the rest of the portfolio largely irrelevant to short-term returns.
How often are these indices rebalanced?
Rebalancing schedules differ by provider and index. Some reconstitute quarterly, others annually, and providers typically announce changes before the effective date. Funds tracking the index then adjust holdings, which can create temporary volume and price effects in the affected stocks.
Does buying an index ETF mean I own every stock equally?
No. Most major index ETFs use market-cap weighting, so larger companies carry larger weights. Equal-weight versions exist, but they rebalance more frequently and can behave very differently from the standard cap-weighted index.
AI Notice: This article was created wholly or predominantly with the assistance of artificial intelligence and was published without human editorial review.
This article is for general information only and does not constitute investment advice. Always do your own research before making trading decisions.