What Is the S&P 500? The Market's Most Important Benchmark

When someone says "the market was up today," they almost always mean the S&P 500. Every fund manager measures performance against it, every financial advisor references it, and every retirement account effectively tracks it. Understanding the S&P 500 isn't optional for investors. It's foundational.

What Is the S&P 500?

The S&P 500 is a stock market index that tracks 500 of the largest publicly traded companies in the United States. It's maintained by S&P Dow Jones Indices (a division of S&P Global) and is widely considered the single best gauge of US large-cap stock performance.

When you hear "the S&P 500 returned 10% this year," that means the combined value of those 500 companies, weighted by their market capitalizations, increased by 10%. You can't buy the index directly, but you can buy funds that replicate it almost perfectly.

How Companies Get In (and Out)

Here's something most people don't know: the S&P 500 is not just the 500 largest US companies. A committee at S&P Dow Jones Indices decides which companies are included. There are eligibility requirements: a company must be US-based, have a market cap above ~$18 billion, be profitable over the most recent four quarters combined, and have adequate liquidity, but meeting the criteria doesn't guarantee inclusion.

The committee meets regularly and makes additions and removals based on their judgment. When a company is added to the S&P 500, index funds tracking it must buy shares, which often pushes the stock price up. When a company is removed, the opposite happens. This "index effect" is real and measurable.

Notable recent additions include companies that replaced others due to mergers, acquisitions, or declining market caps. The index constantly evolves to reflect the current landscape of corporate America.

Market-Cap Weighting: Why It Matters

The S&P 500 is market-cap weighted, which means larger companies have a bigger influence on the index's performance. If Apple (worth ~$3 trillion) goes up 1%, it moves the index far more than a smaller company going up 1%.

This has a massive practical implication: the top 10 companies in the S&P 500 account for more than 30% of the entire index. That means ~490 companies account for less than 70% of your investment. You're not getting 500 equal bets. You're getting a portfolio heavily concentrated in the biggest names.

Market-cap weighting has benefits (you own more of the winners) and drawbacks (you're very exposed to a few mega-cap stocks). It's why some investors supplement S&P 500 holdings with equal-weight or small-cap funds.

The Magnificent Seven Concentration

As of the mid-2020s, the S&P 500's concentration problem has a name: the Magnificent Seven. Apple, Microsoft, NVIDIA, Amazon, Alphabet (Google), Meta, and Tesla have collectively accounted for an outsized share of the index's returns and weight.

At various points, these seven stocks have represented 25-30% of the entire S&P 500's market cap. When people say "the market is up," it often means these seven stocks are up, while hundreds of other companies may be flat or down. This concentration makes the S&P 500 more like a tech-heavy growth fund than the broadly diversified index many investors assume it is.

This isn't necessarily bad. These companies are massively profitable and dominant in their industries. But it means your "diversified" S&P 500 investment carries more single-stock risk than the number 500 suggests.

Historical Returns: The 10% Number

The S&P 500 has returned ~10% per year on average since its inception in 1957 (and similar returns going back further using reconstructed data). This includes price appreciation and reinvested dividends, but not inflation. Adjusted for inflation, the real return is closer to 7% annually.

A few critical caveats about that 10% figure:

• It's a long-term average.In any given year, the S&P 500 has returned anywhere from -37% (2008) to +38% (1995). The average year looks nothing like the "average."
• Sequence of returns matters. Getting -30% in year one and +30% in year two is worse than getting 0% both years (due to compounding math). When you get your returns matters as much as the average.
• Dividends account for ~2% of that 10%.If you don't reinvest dividends, your actual return is significantly lower.
• Past performance doesn't guarantee future results. That said, US large-cap stocks have been one of the better-performing asset classes over virtually every long-term period in modern history.

S&P 500 vs Total Stock Market

The S&P 500 covers ~80% of the total US stock market by market capitalization. A total stock market fund (like VTI or ITOT) includes those same 500 companies plus thousands of mid-cap and small-cap stocks.

In practice, the performance difference between the S&P 500 and the total US stock market is small, often within a fraction of a percent per year. The 500 largest companies dominate both indexes. However, total market funds give you exposure to smaller companies that may outperform over certain periods and provide slightly more diversification.

Which should you choose? Either is fine. If you already own an S&P 500 fund and want small-cap exposure, you can add a dedicated small-cap fund rather than switching. The important thing is being invested, not which flavor of broad US index you pick.

SPY vs VOO vs IVV: Choosing an S&P 500 Fund

There are three main ETFs that track the S&P 500, and they're nearly identical:

• SPY (SPDR S&P 500 ETF Trust):The original, launched in 1993. It's the most heavily traded ETF in the world, which means the tightest bid-ask spreads. Expense ratio: 0.0945%. Slightly higher than its competitors.
• VOO (Vanguard S&P 500 ETF): Launched in 2010 by Vanguard. Expense ratio: 0.03%. Lower fees than SPY. Slightly less trading volume but more than sufficient for any retail investor.
• IVV (iShares Core S&P 500 ETF): Launched in 2000 by BlackRock. Expense ratio: 0.03%. Nearly identical to VOO in cost and structure.

For long-term investors, VOO and IVV are slightly better choices due to lower fees. SPY is preferred by active traders who value its massive liquidity. Over a 30-year holding period, that 0.06% fee difference between SPY and VOO can add up to thousands of dollars on a large portfolio.

The Benchmark Everything Is Measured Against

The S&P 500's most important role in finance isn't as an investment. It's a benchmark. Every actively managed fund is compared to the S&P 500 (or a similar index). And most of them lose.

Over 15-year periods, ~90% of actively managed US large-cap funds underperform the S&P 500 after fees. This is the important statistic in investing. It means that the simplest, lowest-effort strategy, buying an S&P 500 index fund and holding it, beats almost all professionals over the long term.

This is why Warren Buffett famously bet a hedge fund manager $1 million that an S&P 500 index fund would outperform a basket of hedge funds over 10 years. Buffett won easily.

Limitations of the S&P 500

The S&P 500 is excellent, but it's not perfect:

• US-only:It doesn't include any international companies. The US represents ~60% of global stock market value, so an S&P 500-only portfolio misses 40% of the world.
• Large-cap only: No small-cap or mid-cap exposure. Small caps have historically offered higher returns (with higher volatility) over very long periods.
• Concentration risk:As mentioned, a handful of tech giants dominate the index. A tech downturn hits the S&P 500 harder than a truly diversified portfolio.
• Survivorship bias: Companies that decline get removed and replaced by rising companies. The index always looks healthy because it only includes winners.

What to Do Next

The S&P 500 is the starting point for most investment portfolios, not the endpoint. Consider it the foundation: broad US large-cap exposure at minimal cost. From there, you can add international stocks, small-cap stocks, bonds, real estate, or other assets based on your goals and risk tolerance.

Here is the diversification check most people skip: if the top ten S&P 500 holdings already make up 35% of the index, and you also own those same stocks individually, you are far more concentrated than your fund count suggests.