Total Stock Market vs S&P 500: Does the Extra Diversification Matter

Total Stock Market vs S&P 500: Does the Extra Diversification Matter?

Every few months, someone in a personal finance forum posts the same question: should I invest in a total stock market index fund or just stick with the S&P 500? The replies pile up fast, half the people saying it doesn’t matter, the other half acting like the answer is obvious. Neither camp is entirely right, and the real answer requires looking at some actual numbers rather than vibes.

Related: index fund investing guide

I want to walk through this carefully because I’ve seen smart people — engineers, doctors, analysts — make this decision based on incomplete information. The choice isn’t catastrophic either way, but it’s worth understanding what you’re actually getting before you set up an automatic investment and forget about it for 30 years.

What Each Fund Actually Contains

Let’s be precise about what we’re comparing. The S&P 500 tracks 500 of the largest U.S. companies by market capitalization, as selected by a committee at S&P Dow Jones Indices. It covers roughly 80% of the total U.S. stock market by market cap. When people say “the market is up today,” they’re almost always talking about the S&P 500.

A total stock market fund — think Vanguard’s VTI or Fidelity’s FSKAX — tracks the entire investable U.S. equity market, which includes those same 500 large-cap stocks plus thousands of mid-cap and small-cap companies. Depending on the index, you’re looking at somewhere between 3,500 and 4,000 individual stocks.

Here’s the part that surprises most people: because the total market is market-cap weighted, the S&P 500 companies still dominate. The largest 500 companies represent about 80% of the total market’s weight, which means the remaining 3,000+ smaller companies collectively make up only around 20% of a total market fund. You’re not dramatically reshuffling your portfolio by choosing one over the other — you’re making a relatively subtle adjustment to your small- and mid-cap exposure.

The Historical Performance Picture

Over long time horizons, the two have tracked each other remarkably closely. Research from Vanguard has shown that the performance difference between total market funds and S&P 500 funds over 10, 20, and 30-year periods is typically less than 0.5% annually (Wallick et al., 2015). Sometimes the total market wins by a narrow margin, sometimes the S&P 500 does. Neither dominates consistently enough to make a clear case on returns alone.

That said, there are specific periods where small-cap stocks significantly outperformed large-caps. The early 2000s, after the dot-com bubble burst large-cap tech stocks, were a strong period for small- and mid-cap companies. If you held a total market fund during that stretch, you captured more of that recovery than an S&P 500-only investor. Conversely, the 2010s were largely dominated by mega-cap tech, where the S&P 500’s heavier concentration in companies like Apple, Microsoft, and Amazon actually worked in its favor.

This pattern reflects a well-documented phenomenon in financial research. Fama and French (1992) identified what became known as the size premium — the historical tendency for small-cap stocks to outperform large-cap stocks over long periods. Their three-factor model showed that exposure to small-cap value stocks has historically rewarded patient investors. However, this premium has been inconsistent in recent decades, with some researchers arguing it has been arbitraged away as more capital flowed into small-cap index funds.

The Diversification Argument — and Its Limits

From a pure diversification standpoint, owning 4,000 stocks is better than owning 500. That’s not controversial. But diversification only reduces risk when the additional assets aren’t highly correlated with what you already hold. And here’s the problem: U.S. large-caps, mid-caps, and small-caps tend to move together, especially during market crises.

During the 2008-2009 financial crisis, everything fell together. During the COVID crash of March 2020, everything fell together. Small-cap stocks often fall harder during downturns than large-caps because smaller companies tend to have less access to credit, thinner margins, and less diversified revenue streams. So the extra diversification you think you’re getting from 3,000 additional small-cap names doesn’t insulate you from volatility in the way that, say, adding international stocks or bonds would.

This is not an argument against total market funds. It’s an argument for being clear-eyed about what kind of diversification you’re actually adding. You’re getting broader U.S. equity exposure, not a fundamentally different risk profile. If you want genuine diversification that behaves differently from the S&P 500, you need assets outside U.S. large-cap equities altogether — international developed markets, emerging markets, REITs, bonds, or alternatives. [3]

Cost Differences: Smaller Than You Think

Both fund types are extremely cheap at major brokerages. VTI (Vanguard Total Stock Market ETF) carries an expense ratio of 0.03%. VOO (Vanguard S&P 500 ETF) is also 0.03%. Fidelity’s total market and S&P 500 index funds are similarly priced, with some zero-expense-ratio options available. The cost argument that once favored one over the other has essentially collapsed — at this level, the difference is negligible over any realistic investment horizon. [1]

This is worth emphasizing because the expense ratio battle was real 20 years ago. Retail investors were paying 1-2% annually on actively managed funds, and the move to index investing was genuinely transformative in terms of wealth accumulation over time. Bogle (2010) documented extensively how expense ratios compound against investors over time in ways that are deeply underappreciated. But when comparing two similarly structured index products at 0.03%, this consideration essentially drops out of the equation. You’re not making a meaningful financial error either way based on costs alone. [2]

Tax Efficiency and Turnover

For investors holding funds in taxable brokerage accounts — not just 401(k)s and IRAs — there’s another angle worth considering: tax efficiency. Index funds generally have low turnover, which means fewer taxable capital gains distributions. Both total market and S&P 500 index funds are excellent on this dimension compared to actively managed funds. [4]

[5]

The S&P 500 does have slightly more turnover than a pure total market fund because the S&P 500 is committee-selected rather than rules-based. When a company is added to or removed from the S&P 500 index, the fund must trade. A total market fund based on something like the CRSP US Total Market Index follows more mechanical rules, which can result in somewhat less turnover. In practice, the difference is minimal for most investors, but if you’re highly tax-sensitive and investing large sums in a taxable account, it’s a factor worth noting.

Asset location strategy — the practice of holding tax-inefficient assets in tax-advantaged accounts and tax-efficient assets in taxable accounts — is generally more impactful than choosing between these two fund types (Horan & Adler, 2009). If you have both types of accounts, thinking carefully about which assets go where will likely do more for your after-tax returns than the fund selection itself.

The Small-Cap Premium: Real or Residual?

Let’s spend more time on this because it’s genuinely contested. The original Fama-French research found that small-cap stocks historically generated higher returns than large-cap stocks, even after adjusting for market risk. The theoretical explanation involves compensation for additional risks — smaller companies are less liquid, more vulnerable to economic cycles, and carry higher bankruptcy risk. Investors demand a higher expected return for bearing those risks.

But since that research was published and became widely known, a few things have happened. First, massive inflows into small-cap index funds may have reduced the premium by bidding up small-cap prices. Second, the premium has been much weaker or absent in the U.S. market since the 1980s. Third, some researchers have argued the original findings partially reflected data mining, and that the premium was never as robust as the initial studies suggested (Harvey et al., 2016).

What this means practically: you shouldn’t choose a total market fund over an S&P 500 fund specifically because you’re expecting small-cap outperformance to compensate you for the difference. That bet has not paid off reliably. The case for total market funds rests more on completeness — owning the whole market rather than a large slice of it — than on expecting small-cap stocks to pull your returns higher.

Behavioral Considerations for ADHD-Prone Investors

Speaking from personal experience here, and I mean that literally. When you manage attention difficulties, the number of moving pieces in a portfolio matters. Every additional decision point is a potential source of second-guessing, tinkering, and suboptimal action taken during market stress.

One of the strongest arguments for either of these funds over more complex strategies is their simplicity. You buy one fund, you get broad exposure, you continue contributing, you don’t check it every day. The behavioral finance literature consistently shows that investor returns lag fund returns because people make poor timing decisions — buying after markets have risen and selling after they’ve fallen (Barber & Odean, 2000). The gap between what a fund earns and what the average investor in that fund actually earns can be several percentage points annually.

From this perspective, the best fund is the one you’ll actually stay invested in during a 30-40% drawdown. If the simplicity of “I own the S&P 500, the largest 500 American companies” helps you hold through volatility, that psychological clarity has real economic value. If you find the total market framing more satisfying — “I own the entire U.S. stock market” — that works just as well. The difference in outcomes from the fund choice itself is small compared to the outcome difference between staying invested and panic-selling.

International Exposure: The Bigger Missing Piece

Whatever you decide about total market vs. S&P 500, there’s a more significant diversification question lurking underneath: U.S.-only vs. global exposure. The U.S. stock market represents roughly 60% of global market capitalization, which means a U.S.-only investor is making an active bet against the other 40% of the world’s publicly traded companies.

Historically, that bet has paid off well for the past 15 years — U.S. markets have dramatically outperformed international markets since roughly 2010. But leadership rotates. The 2000s were a period when international stocks outperformed U.S. stocks significantly. Holding a globally diversified portfolio smooths these cycles, though it also means you’ll sometimes underperform the U.S.-only benchmark during American bull markets.

The point isn’t to tell you what to do about international allocation — that’s a separate conversation and depends on your beliefs about future relative performance, currency risk tolerance, and how much tracking error you can psychologically stomach. But it’s worth noting that if you’re deeply focused on the total market vs. S&P 500 question, you may be optimizing a small variable while ignoring a larger one. The spread between total market and S&P 500 outcomes over 30 years is likely to be measured in fractions of a percent annually. The spread between U.S.-only and globally diversified outcomes could be much larger in either direction.

So Which One Should You Actually Pick?

Both are excellent choices and you’re not making a mistake with either one. But if forced to give a preference, here’s how I think about it: if you’re building a simple, single-fund U.S. equity position, the total market fund is slightly more theoretically complete. You own the market, not a committee-selected subset of it. The rules-based construction avoids the small reconstitution costs that come with S&P 500 index changes. And you capture small- and mid-cap exposure, even if that exposure doesn’t dramatically change your expected returns.

If you’re already working with a three-fund or four-fund portfolio that includes international equities and a bond allocation, the distinction matters even less. Your overall asset allocation will dominate your investment outcomes far more than whether you chose VTI or VOO for your U.S. equity sleeve.

The one scenario where the S&P 500 fund might make slightly more sense is if you’re investing in a workplace retirement plan with limited fund options. In that context, you take what you can get at a low cost, and the S&P 500 index fund is typically a solid, low-cost option that covers the vast majority of U.S. market exposure. Chasing a total market fund when a perfectly good S&P 500 option is available is not worth losing sleep over.

What actually moves the needle on your long-term wealth accumulation is your savings rate, your asset allocation between stocks and bonds, your willingness to stay invested during downturns, and minimizing costs and taxes where possible. The gap between total market and S&P 500 funds is genuinely small relative to any of those factors. Pick one, automate your contributions, and direct your analytical energy toward things that have larger effects on your financial future.

Last updated: 2026-03-31

References

• Frait, Eric (2024). Building a Better Market Index. Chicago Booth Magazine. Link
• Kritzman, Mark and Turkington, David (2025). The Fallacy of Concentration. Working Paper. Link
• CRSP (n.d.). What “Owning the Market” Really Means. CRSP. Link
• Commonfund (2025). The New Era of Market Concentration. Commonfund Blog. Link
• J.P. Morgan Private Bank (2025). Why the U.S. economy and S&P 500 are diverging. J.P. Morgan. Link

Related Reading

• What Is a REIT and How to Invest in Real Estate
• What Is a Bond and How It Works
• The Small Cap Value Premium: 97 Years of Data Most Investors Miss