Wealth management
Understanding the S&P 500—what it means for you and your portfolio
While the S&P 500 anchors many portfolios as a trusted U.S. stock benchmark, understanding what it includes—and critically, what it doesn’t—reveals important gaps in diversification.
What is the “S&P”
If you follow financial news, you hear about the S&P 500 constantly. One day the S&P 500 is up, another it’s down. Business news breaks—tariffs, rate cuts, jobs data, etc.—and the first thing pundits often want to talk about is the impact it might have on the “S&P.”
This ubiquitous equity index has become a daily barometer of how the stock market and economy are performing. But what exactly is the S&P 500, and why does it matter so much to your investments?
“The S&P 500 is, in many ways, a proxy for the overall U.S. equity market,” explains Michael Sowa, deputy chief investment officer for TIAA Wealth Management. “It’s constructed of U.S.-based companies, and it’s weighted based on the size of each company—what we call market capitalization.”
Understanding the S&P 500 is important because it’s not just a benchmark that financial professionals use to measure performance. Thanks to the growth of index investing, it’s also become one of the largest investment vehicles in the world. Today, approximately $16 trillion is invested in S&P 500-based index mutual funds and exchange-traded funds (ETFs),1 making this index a cornerstone of many investors’ portfolios—including, quite possibly, yours.
But here’s what many investors don’t understand: While the S&P 500 provides important exposure to the U.S. stock market, it doesn’t offer the comprehensive diversification that many people assume. You’re not getting international stocks. You’re not getting smaller U.S. companies. And because of how the index is constructed, you may have far more concentration in a handful of mega-cap technology stocks than you realize, especially if you own other tech investments alongside your S&P 500 holdings.
A brief history
The S&P 500 was introduced by Standard & Poor’s in March 1957. The index was designed to capture approximately 80% of the total market value of all publicly traded U.S. companies, making it far more representative than other popular indices of the time.
What made the S&P 500 innovative was its market capitalization-weighted methodology, which means that larger companies have a proportionally larger influence on the index’s performance. This approach reflects how most investment portfolios are actually constructed and remains the calculation method today.
How companies make the cut
Getting into the S&P 500 isn’t simply about being among the 500 largest U.S. companies. The Index Committee at S&P Dow Jones Indices uses specific criteria to select companies, including minimum market capitalization requirements (currently $22.7 billion), adequate trading liquidity, and positive earnings. The committee also considers sector balance to ensure the index reflects the overall composition of the U.S. economy.2
The index is dynamic, with typically 20 to 30 changes occurring annually due to mergers, acquisitions, or companies no longer meeting the eligibility requirements.
Why it matters as a benchmark and investment
The S&P 500 has become the gold standard for measuring U.S. stock market performance for several reasons. It provides broad exposure across all 11 sectors of the economy—from technology and health care to consumer staples and utilities. Its market-cap weighting means it reflects how the market actually behaves. And because it comprises highly liquid, financially viable companies, it’s both practical as a benchmark and as something people can invest in via index funds and ETFs.
“The S&P 500 today is important to clients and investors because of the sheer growth potential of the U.S. markets,” says Sowa. “It provides diversification across market capitalization and across all 11 sectors of the U.S. market.”
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What the S&P 500 doesn’t give you
Here’s what many investors don’t fully appreciate though: The S&P 500, while diversified across U.S. sectors, doesn’t provide as much portfolio diversification as you might think.
First, it’s heavily concentrated in whatever sectors are currently dominant. Thirty years ago, financial companies had the biggest weighting. Today, it’s technology. “The composition of the S&P 500 has changed over time based on the market capitalization of certain sectors and industries,” Sowa explains. “Today, it’s very heavily weighted in technology, and even more recently, greater weight has been attributed to the MAG 7”—the seven mega-cap technology stocks (Apple, Microsoft, Amazon, Alphabet, Meta, Nvidia, and Tesla) that have driven much of the market’s recent performance.
Second, you’re not getting international exposure. “Think of all those international brands domiciled in Europe, Japan, or in emerging markets,” says Sowa. “You’re not getting access to those companies, and those are companies that have great growth potential and attractive valuations.”
Third, you’re missing smaller U.S. companies. The S&P 500 focuses on large, established corporations. Small-cap stocks, which historically have provided both growth potential and diversification benefits, are not included.
The danger of unintended concentration
Because the S&P 500 is market-cap weighted, investors need to be mindful of how much technology exposure they’re carrying in their portfolio. If you own an S&P 500 index fund and also invest in individual tech stocks or a technology sector ETF, you may be doubling down on the same companies without realizing it.
“If you’re going to invest directly into some of the underlying sectors of the S&P 500, you’re going to be at risk of over-allocating,” Sowa cautions.
Bottom line: If you own technology funds in addition to S&P 500 funds, you could end up with far more concentration in a handful of mega-cap tech stocks and far less diversification.
Building a truly diversified portfolio
At TIAA Wealth Management, Sowa and his colleagues think about diversification more broadly when assembling
This approach helps explain why managed accounts might not always match the S&P 500’s performance in any given year. “In periods where the S&P 500 is outperforming other segments of our diversified portfolio, our diversified portfolio is going to lag, and vice versa,” Sowa says. “That’s by design.”
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1 Alex Mayyasi, “The surprisingly lucrative business of making a list of 500 stocks,” Planet Money, NPR, September 23, 2025.
2 S&P Dow Jones Indices, “S&P U.S. Indices Methodology,” February 2026.
The views expressed in this material may change in response to changing economic and market conditions. Past performance is not indicative of future returns.
No strategy can eliminate or anticipate all market risks, and losses can occur.
The TIAA group of companies does not provide tax or legal advice. Tax and other laws are subject to change, either prospectively or retroactively. Individuals should consult with a qualified independent tax advisor and/or attorney for specific advice based on the individual’s personal circumstances.