The S&P 500 is the benchmark most investors use when discussing the US stock market. It contains 500 leading US companies and covers a large majority of the country's listed equity value. Yet it is not a simple list of the 500 largest stocks, and each company does not receive an equal weight.
S&P Dow Jones Indices uses eligibility criteria that include market capitalisation, liquidity, public float, profitability and US domicile. An index committee makes final inclusion decisions. That means a company can be very large without joining immediately.
Market-cap weighting drives concentration
The index is weighted by float-adjusted market capitalisation. A company worth several trillion dollars therefore has much more influence than a company worth tens of billions. Nvidia, Microsoft, Apple, Alphabet, Amazon and Meta can collectively account for a substantial share of the index.
This structure has advantages. Investors automatically own more of companies that the market values more highly. The drawback is concentration. A broad index fund can still behave like a portfolio with a large technology allocation when the biggest technology stocks dominate market value.
Sector weights change over time
Information technology is one of the largest S&P 500 sectors, but the classification system also places some major digital businesses elsewhere. Alphabet and Meta are classified in communication services, while Amazon and Tesla sit in consumer discretionary.
That makes headline sector weights less intuitive than they appear. Investors analysing concentration should look at individual holdings as well as formal sector labels.
How companies enter and leave
The index committee replaces companies after acquisitions, failures, spin-offs and other corporate events. New entrants must satisfy eligibility rules, while companies can be removed when they no longer meet requirements or when an index change is needed.
Inclusion can matter because index funds tracking the S&P 500 need to buy the new constituent. The long-term impact is usually more important for liquidity and ownership than for fundamental value, but announcement-day price moves can be significant.
Why the S&P 500 remains useful
Despite concentration, the index remains a powerful benchmark because it is transparent, investable and diversified across hundreds of large companies. It also has a long history that makes performance comparisons straightforward.
Investors should simply understand what they own. Buying the S&P 500 is not the same as buying equal exposure to the US economy. It is buying a market-cap weighted portfolio of large listed companies, with the winners receiving the largest weights.