An S&P 500 index fund is often described as a simple way to own 500 large American companies. That description is correct, but incomplete.

The fund does not divide your money equally among those companies. The largest businesses receive the largest allocations. By late July 2026, the seven companies commonly called the Magnificent Seven represented roughly one-third of the S&P 500. (1)

This concentration does not automatically make the index unsafe or unattractive. It does mean that investors should understand where their money is allocated, how the index’s return is calculated and which risks they are actually accepting.

This article explains:

  • How the S&P 500 decides how much to invest in each company
  • How the index can rise while some of its largest companies fall
  • How to measure whether a rally is broad or narrow
  • How credit conditions and market valuations should be interpreted
  • How investors can reduce concentration if it exceeds their intended level

Data in this article are dated because market prices and index weights change every trading day.

1. How an S&P 500 index fund allocates your money

The S&P 500 is an index: a set of rules used to measure the performance of approximately 500 large US companies.

An index fund is a fund designed to copy those rules. An exchange-traded fund, or ETF, is a fund whose shares can be bought and sold on a stock exchange.

The S&P 500 uses float-adjusted market-capitalisation weighting. This lengthy term has a simple meaning.

A company’s market capitalisation is calculated as:

\[\text{market capitalisation} = \text{share price} \times \text{number of shares}\]

“Float-adjusted” means that the calculation generally includes shares available to public investors while excluding certain tightly held shares, such as some shares owned by founders, governments or controlling shareholders. (2)

A company’s approximate weight in the index is then:

\[\text{company weight} = \frac{\text{company's float-adjusted market value}} {\text{total float-adjusted market value of the index}}\]

Consider a simplified example:

  • Company A has a float-adjusted market value of \$4 trillion.
  • All companies in the index together have a float-adjusted market value of \$60 trillion.

The calculation is:

\[\text{Company A's weight} = \frac{\$4\text{ trillion}}{\$60\text{ trillion}}\] \[\text{Company A's weight} = 0.0667\]

Convert that decimal into a percentage:

\[0.0667 \times 100 = 6.67\%\]

An investor with \$100,000 in the fund would therefore have approximately:

\[\$100{,}000 \times 6.67\% = \$6{,}670\]

allocated to Company A.

What one-third in seven companies means

In late July 2026, the Magnificent Seven, Alphabet, Amazon, Apple, Meta, Microsoft, Nvidia and Tesla, represented roughly one-third of the S&P 500. These seven companies are a well-known group, although they are not always literally the seven largest securities in the index. Reuters reported that their combined weight remained around one-third of the S&P 500. (1)

Using a 33% group weight, a \$100,000 investment can be divided as follows:

Amount in the Magnificent Seven

\[\$100{,}000 \times 33\% = \$33{,}000\]

Amount in the remaining companies

\[\$100{,}000 - \$33{,}000 = \$67{,}000\]

The investor owns hundreds of companies, but approximately one dollar in every three depends on seven of them.

That is diversification by number of companies, combined with concentration by amount of money.

2. How the index can rise while its largest companies struggle

An index return is a weighted average.

A weighted average gives more importance to the companies with the largest index weights. The basic formula is:

\[\text{index return} = \sum_i \left(\text{company }i\text{'s weight} \times \text{company }i\text{'s return}\right)\]

For a simplified two-group example, suppose:

  • The largest group has a 33% weight and falls by 3%.
  • The rest of the index has a 67% weight and rises by 13%.

First calculate the large group’s contribution:

\[33\% \times (-3\%) = -0.99 \text{ percentage points}\]

A percentage point is the direct difference between two percentage numbers. Here, the large group subtracts 0.99 percentage points from the index return.

Now calculate the rest of the index:

\[67\% \times 13\% = 8.71 \text{ percentage points}\]

Add the two contributions:

\[-0.99 + 8.71 = 7.72\%\]

The overall index rises by approximately 7.72%, even though its largest group falls.

Nothing has disappeared from the calculation. The stronger return from the larger collection of other companies has more than offset the loss from the concentrated group.

A late-July 2026 example

On 23 July 2026, one useful market snapshot showed:

  • MAGS, an ETF that gives equal weight to the Magnificent Seven, down approximately 3.7% for the year
  • XMAG, an ETF covering a broad group of large US companies while excluding the Magnificent Seven, up approximately 12.9%
  • The S&P 500 up approximately 8.2%

These funds use different construction methods, so their returns should be treated as measures of market leadership rather than as an exact mathematical breakdown of the S&P 500. Nevertheless, the gap showed that the broader market was performing much better than the seven-company basket at that point. The figures were reported on 24 July 2026. (3)

By the market close on 31 July, the S&P 500’s year-to-date return had changed again:

  • Price return: 9.41%
  • Dividend contribution: 0.73%
  • Total return: 10.14%

The calculation is:

\[9.41\% + 0.73\% = 10.14\%\]

A price return measures only the change in the index level. A total return also includes dividends paid by the companies, assuming those dividends are reinvested. Slickcharts reported these figures through 31 July 2026. (4)

This is why every return figure should state both its date and whether it is a price return or total return.

3. Concentration changes both gains and losses

Concentration means that a large part of a portfolio depends on a small number of investments.

Concentration is not automatically good or bad. Its effect depends on what happens to the companies receiving the largest weights.

If those businesses continue growing their earnings, the concentration can improve the index’s return. If they disappoint investors, their large weights can pull down the whole index.

The Magnificent Seven are not speculative companies without established businesses. They include some of the world’s largest and most profitable companies. Their size is supported by substantial revenue, earnings and cash generation.

The risk comes from dependence, not from assuming that the companies are weak.

An investor in a market-capitalisation-weighted fund is especially dependent on:

  • The future earnings of a small group of very large companies
  • The prices investors are willing to pay for those earnings
  • Shared business themes such as artificial intelligence, cloud computing, digital advertising and semiconductor investment
  • Market expectations about interest rates and future economic growth

Market-capitalisation weighting also allows concentration to grow naturally. If a company’s share price rises faster than the rest of the index, its weight increases automatically. The fund does not necessarily need to buy additional shares merely because the price rose; the existing shares have simply become more valuable.

This system has an important benefit: successful companies receive larger weights without a fund manager needing to predict the winners in advance. Its weakness is that the fund can become heavily exposed to companies after their prices have already risen significantly.

4. Market breadth shows how widely returns are distributed

Market breadth describes how many companies are participating in a market rise or fall.

A broad market rise involves many companies. A narrow rise depends mainly on a small group.

Three useful measures are:

Capitalisation-weighted S&P 500

This is the standard S&P 500. Larger companies have greater influence over its return.

Equal-weight S&P 500

This index contains the same S&P 500 companies but gives each company approximately the same weight at its quarterly rebalancing.

Rebalancing means resetting the portfolio to its target weights.

With approximately 500 companies, the starting weight is roughly:

\[100\% \div 500 = 0.2\% \text{ per company}\]

The seven companies therefore receive approximately:

\[7 \times 0.2\% = 1.4\%\]

at each quarterly reset.

The official index uses the same constituent companies as the standard S&P 500 but changes how much influence each company receives. (5)

Russell 2000

The Russell 2000 measures approximately 2,000 smaller US-listed companies. A small-cap company is a company with a smaller market value than the large businesses that dominate the S&P 500.

The Russell 2000 is a separate small-company index. It is not simply the S&P 500 with the Magnificent Seven removed. (6)

What the 2026 figures showed

By 29 July 2026:

  • The standard S&P 500 was up approximately 8.5% for the year.
  • The equal-weight S&P 500 was up more than 13%.
  • Small-cap US stocks were up approximately 19%.

The equal-weight index’s advantage over the standard index was therefore more than:

\[13\% - 8.5\% = 4.5 \text{ percentage points}\]

The small-cap advantage was approximately:

\[19\% - 8.5\% = 10.5 \text{ percentage points}\]

These differences showed that market gains had spread beyond the largest companies. Reuters reported the broadening in market performance on 29 July 2026. (1)

This is sometimes called a market rotation. A rotation means that one part of the market begins outperforming another. It does not require an equal amount of cash to move directly from one group to the other. Stock-market value can be created or lost as prices change.

Market breadth describes what is happening across stocks. It does not, by itself, predict whether the whole market will rise or fall next.

5. Credit conditions help measure economic stress

Stock prices show what investors are willing to pay. Credit conditions show how easy or difficult it is for households and businesses to borrow money.

Credit matters because many companies depend on borrowing to finance equipment, buildings, stock, acquisitions and day-to-day operations. When banks make loans harder to obtain, weaker companies can come under pressure.

The Federal Reserve measures bank-lending conditions through the Senior Loan Officer Opinion Survey on Bank Lending Practices, commonly shortened to SLOOS.

One widely followed series measures the net percentage of banks tightening lending standards for commercial and industrial loans to large and medium-sized businesses.

“Net percentage” is calculated as:

\[\text{net percentage} = \text{percentage tightening} - \text{percentage easing}\]

Suppose:

  • 20% of banks tighten their lending standards.
  • 12% ease their lending standards.

The reported net figure is:

\[20\% - 12\% = 8\%\]

This does not mean that only 8% of banks tightened. It means that the tightening percentage exceeded the easing percentage by eight percentage points.

For the second quarter of 2026, the reported figure was 8.1%. That was higher than the first-quarter reading of 5.3%, but far below the extreme levels reached during severe credit disruptions. The complete quarterly series is available from the Federal Reserve Bank of St. Louis. (7)

How to interpret the survey

There is no single number that reliably announces a market crash.

The survey is more useful when investors examine:

  • The current level
  • Whether the figure is rising or falling
  • How quickly it is changing
  • Whether tightening is spreading across several types of loans
  • Whether demand for loans is also weakening

For example, the relevant reading was approximately 19.2% around the October 2007 stock-market peak and rose above 50% during 2008. In 2020, it moved from approximately zero to above 40% only after the sudden pandemic shock had begun. (7)

Credit data therefore provide evidence about financial and economic pressure. They are not a precise market-timing clock.

6. Valuation tells us how much investors are paying

A market can have broad participation and easy credit while still being expensive.

One widely followed valuation measure is the Shiller cyclically adjusted price-to-earnings ratio, usually called the Shiller CAPE ratio.

A normal price-to-earnings ratio compares a company’s share price with one year of earnings. CAPE uses ten years of inflation-adjusted earnings to reduce the effect of unusually strong or weak individual years.

The formula is:

\[\text{CAPE} = \frac{\text{index price}} {\text{average inflation-adjusted earnings over the previous ten years}}\]

Suppose:

  • The index price is 4,000.
  • Average inflation-adjusted earnings are 100.

The calculation is:

\[4{,}000 \div 100 = 40\]

A CAPE of 40 means investors are paying approximately \$40 for every \$1 of average ten-year earnings.

By late July 2026, the US market’s CAPE was close to 40-41. The dot-com-era peak was approximately 44, while the very long-run average is close to 17. Professor Robert Shiller publishes the underlying historical data through Yale University, and the 31 July 2026 monthly reading was 40.91. (8, 9)

A high CAPE has two important meanings:

  • Share prices are high relative to long-term earnings.
  • Future long-term returns are likely to depend heavily on continued earnings growth.

It does not mean the market must fall immediately. Expensive markets can remain expensive for years, especially when large companies continue increasing their profits.

Research generally finds that valuation is more useful for estimating returns over long periods than for identifying the exact date of a market decline. (10)

7. Four ways to manage S&P 500 concentration

There is no single correct allocation for every investor. The appropriate choice depends on objectives, time horizon, costs, tax position and willingness to accept periods of underperformance.

Choice 1: Keep the standard S&P 500 fund

This preserves:

  • Very low fees
  • Automatic exposure to successful growing companies
  • Low maintenance
  • Broad ownership of large US businesses

The trade-off is accepting that roughly one-third of the investment may depend on seven companies.

This can be a deliberate allocation. The important question is whether the concentration matches the investor’s intended risk.

Choice 2: Use an equal-weight S&P 500 fund

Equal weighting reduces the Magnificent Seven from roughly one-third of the portfolio to approximately 1.4% at each quarterly rebalancing.

It also increases exposure to the smaller companies inside the S&P 500.

The trade-offs include:

  • Higher fees
  • More trading when the fund resets its weights
  • Greater exposure to smaller and sometimes less profitable companies
  • Likely underperformance during periods when the largest companies dominate market returns

For example, the Invesco S&P 500 Equal Weight ETF, known by the ticker RSP, charges approximately 0.20% annually. Vanguard’s standard S&P 500 ETF, VOO, charges approximately 0.03%. An expense ratio is the annual fund charge expressed as a percentage of the money invested. (11, 12)

For a \$100,000 investment:

RSP’s approximate annual fee

\[\$100{,}000 \times 0.20\% = \$200\]

VOO’s approximate annual fee

\[\$100{,}000 \times 0.03\% = \$30\]

Approximate annual difference

\[\$200 - \$30 = \$170\]

This calculation isolates the fund charges. It does not include differences in investment performance, trading costs or taxes.

Choice 3: Add a small-company fund

An investor can retain a standard S&P 500 fund while adding a Russell 2000 or another small-company fund.

This reduces the percentage of the total portfolio controlled by the largest companies without removing them.

The trade-off is that smaller businesses can have:

  • Less stable profits
  • More dependence on borrowing
  • Higher share-price volatility
  • Greater sensitivity to changes in the economy

Directing new contributions to smaller companies changes the allocation gradually. It does not immediately alter the money already invested in the S&P 500.

Choice 4: Diversify beyond US shares

Changing from a capitalisation-weighted S&P 500 fund to an equal-weight or small-company fund changes the distribution of US equity risk. It does not remove the risk of the US stock market as a whole.

A more widely diversified portfolio may also include:

  • Companies from countries outside the United States
  • Government or company bonds
  • Cash or short-term government securities
  • Other assets selected for a specific investment purpose

These assets have different risks and expected returns. Their role should be determined by the portfolio’s objective rather than by recent performance alone.

8. Tax and account type matter before changing funds

Buying a new fund and selling an existing fund are financially different actions.

Using new contributions may change the portfolio without selling current holdings. Replacing an existing fund requires a sale, which may create a tax consequence depending on the country and account type.

In the United States, selling fund shares in a taxable brokerage account can create a taxable capital gain or a capital loss that may be deductible. Transactions inside many retirement accounts ordinarily do not create an immediate capital-gains tax bill; for example, amounts in a traditional IRA, including earnings and gains, generally are not taxed until they are distributed. (13, 14)

In the United Kingdom, selling shares or fund units outside an Individual Savings Account, or ISA, may create a disposal for Capital Gains Tax. A disposal is a sale or other transaction treated as giving up ownership of an asset. Investments held inside an ISA are generally protected from UK Income Tax and Capital Gains Tax. (15)

Tax treatment depends on personal circumstances and can change. It should be checked before an existing position is sold.

9. A practical concentration review

An investor can review an index position through five separate questions.

What do I own?

Find the fund provider’s holdings page and examine the percentage held in its largest ten companies.

How much concentration do I have?

Convert the percentage into money:

\[\text{portfolio value} \times \text{group weight} = \text{money allocated to the group}\]

For example:

\[£200{,}000 \times 33\% = £66{,}000\]

Is that concentration intentional?

Decide whether the allocation matches the portfolio’s objective rather than judging it solely from recent returns.

What would a change cost?

Include:

  • Fund fees
  • Trading costs
  • Tax consequences
  • The possibility that the new allocation underperforms

What rule will control future changes?

A portfolio should have a planned method for reviewing and restoring its intended allocations. This might be once or twice a year, or when an allocation moves beyond a predetermined range.

Without a rule, investors can repeatedly move towards whichever part of the market has performed best most recently.

Conclusion

An S&P 500 index fund provides broad access to large American companies, but it does not divide money equally among them. By late July 2026, roughly one-third of the index depended on seven companies.

That concentration can produce strong returns when the largest companies lead. It can also allow the headline index return to conceal very different results among the companies underneath it.

Market breadth helps investors see how widely gains are distributed. Bank-lending surveys help measure credit pressure. CAPE provides long-term valuation context. None of these measures, by itself, predicts the date or size of the next market decline.

The central task is therefore not to guess the next crash. It is to understand the portfolio in money terms, decide how much concentration is appropriate and choose an allocation deliberately.

An investor does not merely own an index number. The investor owns companies, and the amount invested in each company matters.

References

  1. Reuters, “Magnificent 7 results set to test broadening US stock market”
  2. S&P Dow Jones Indices, “Methodology Matters”
  3. Benzinga, “S&P 493 Hammers Magnificent Seven in the Year of the Underdog”
  4. Slickcharts, “S&P 500 Year to Date Return for 2026”
  5. S&P Dow Jones Indices, “S&P 500 Equal Weight Index”
  6. FTSE Russell, “Russell 2000 Index”
  7. Federal Reserve Bank of St. Louis, “Net Percentage of Domestic Banks Tightening Standards for Commercial and Industrial Loans to Large and Middle-Market Firms”
  8. Robert J. Shiller, “Online Data”
  9. Multpl, “Shiller PE Ratio by Month”
  10. AQR Capital Management, “Market Timing: Sin a Little”
  11. Invesco, “Invesco S&P 500 Equal Weight ETF”
  12. Reuters, “Vanguard index product becomes first ETF to top \$1 trillion in assets”
  13. Internal Revenue Service, “Publication 550: Investment Income and Expenses”
  14. Internal Revenue Service, “Publication 590-B: Distributions from Individual Retirement Arrangements”
  15. HM Revenue & Customs, “Tax when you sell shares”