In his 2013 annual letter to Berkshire Hathaway shareholders, Warren Buffett laid out the instructions he had left for the trustee handling the cash his wife will inherit: ten percent in short-term government bonds, and ninety percent in a very low-cost broad-market index fund. Buffett wrote that he believed the long-term results from this simple policy would be superior to those attained by most investors who employ high-fee managers. Coming from an investor whose firm built one of the largest equity portfolios in the world through hands-on stock picking, that recommendation deserves a careful look. This article walks through what an index fund actually is, why fees and structure matter so much, and where the historical evidence sits.
What Is an Index Fund?
An index fund is a mutual fund or exchange-traded fund whose stated goal is to mirror, as closely as possible, the performance of a specified market index. It does this by holding all of the index's component securities, or a representative sample, in roughly the same proportions as the index itself. There is no portfolio manager trying to outguess the market. The fund simply tracks. The first retail index fund of this kind launched in 1976 under John Bogle at Vanguard, originally tracking the S&P 500. Initial reception was hostile — the financial industry derided the idea — but the fund and its descendants slowly accumulated trillions of dollars in assets over the following decades.
Why the Index Concept Works
The theoretical case for indexing rests on two pillars. The first is William Sharpe's 1991 arithmetic argument: in any market, the average actively managed dollar must, by mathematical identity, earn the same gross return as the market average — and therefore less than the market average after costs. The second pillar is the empirical record. The SPIVA Scorecard, published twice yearly since 2002, repeatedly shows that over fifteen-year horizons roughly 85 to 95 percent of actively managed large-cap US equity funds underperform their benchmarks after fees. Similar patterns appear in international SPIVA reports. The arithmetic and the data tell the same story: in a competitive market, the average dollar paying high fees finishes behind the average dollar paying low fees.
The Power of Low Fees
A typical broad-market index fund today charges between three and ten basis points annually — about three to ten dollars per ten thousand invested. A typical actively managed equity mutual fund charges between sixty basis points and well over one percent. The arithmetic of compounding makes that gap brutal over decades. On a thirty-year horizon at an eight percent gross return, a one-percentage-point annual fee differential consumes roughly a quarter of the final portfolio value. The investor's lifestyle in retirement is materially different in the two cases — purely from the fee, before any selection skill is even considered.
Tax Efficiency
Index funds also tend to be more tax-efficient than active funds in taxable accounts. They trade infrequently, generating fewer realized capital gains. ETF structures, in particular, use an in-kind creation and redemption process that further reduces taxable distributions. Over long periods, after-tax returns can run roughly 0.3 to 1 percentage point higher than otherwise similar active funds, depending on jurisdiction and tax bracket. In tax-advantaged accounts, this advantage is muted, but in taxable accounts it stacks on top of the fee advantage.
Diversification by Construction
A total stock market index fund, by definition, holds essentially the entire investable equity market of its target country, weighted by market capitalization. A global all-cap index fund extends this across thousands of companies in dozens of countries. The result is a level of diversification that would be tedious and expensive to assemble manually through individual stock purchases. The single-stock risk is structurally minimized — no individual company failure can permanently damage the portfolio, because the index simply rebalances out of failed companies and into successful ones.
Common Index Fund Types
Total US stock market funds aim to track essentially every publicly listed US company. S&P 500 funds focus on the 500 largest US companies, which together represent the bulk of US equity market capitalization. Total international funds capture non-US developed and emerging markets. Total bond market funds track broad investment-grade fixed income. Sector-specific funds, factor-tilted funds, and ESG-screened funds also exist, but the closer you move from broad index toward narrow theme, the closer you move back toward something that behaves like active management with index-fund branding.
What Index Funds Are Not
Index funds do not protect investors from market drawdowns. When the market falls, an index fund falls with it. The 2008 crash, the 2020 pandemic crash, and the 2022 rate shock all hit index funds in proportion to the underlying index. The advantage of indexing is structural and cumulative over decades, not protective in any single year. Investors who confuse low cost with low risk frequently sell at the wrong time. Index funds are also not all created equal — tracking error, expense ratio, securities lending policy, and underlying index methodology vary across providers, and worth a careful read of the fund prospectus.
Common Mistakes
The first common mistake is selecting an index fund based on a flashy theme rather than broad market exposure. A thematic fund tracking a narrow industry index can experience drawdowns far larger than a total market fund. The second is paying high fees for an index fund — there are providers that charge multiples of the broad-market norm for essentially identical exposure. The third is constantly switching between similar index funds in pursuit of small differences, generating tax events that destroy the underlying advantage. The fourth is using leveraged or inverse index ETFs as long-term holdings; their daily-reset structure makes long-term returns deviate sharply from the simple index path.
Real-World Example
Consider a hypothetical investor contributing five hundred dollars per month over thirty years into a broad US equity index fund with an expense ratio of four basis points. At an eight percent average annual gross return, the final portfolio is materially larger than what the same contributions would produce in an actively managed fund charging one hundred basis points and underperforming the index by another fifty basis points on average. The total fee and selection drag is unimpressive year by year, but over thirty years it compounds into a difference measured in the hundreds of thousands of dollars. The numbers and assumptions are illustrative; past performance does not guarantee future results.
Frequently Asked Questions
Does indexing only work in efficient markets? Indexing tends to be most reliably superior in highly competitive, well-researched markets like US large-cap equities. In less efficient corners — small-cap, frontier emerging markets, certain credit segments — the average active dollar still lags after fees, but the dispersion between managers is wider.
Is there a risk that indexing becomes too popular? Some commentators argue that very high index ownership could distort price discovery. To date, however, active management still represents a large share of trading volume globally, and the academic consensus is that we are not near any pathological tipping point.
How do I choose between mutual fund and ETF versions of the same index? In taxable accounts, ETF structures often offer modest tax advantages. In tax-advantaged accounts, the choice is mostly about cost, accessibility, and convenience.
Is the S&P 500 enough on its own? It covers the majority of US equity capitalization but excludes small-caps and entirely excludes non-US markets. Many educational frameworks suggest a more globally diversified mix.
Key Takeaway
Index funds will not make anyone rich overnight. What they offer is a structural, cumulative advantage that compounds quietly over decades — low fees, broad diversification, tax efficiency, and a behavior-friendly design. As Buffett wrote in his 2013 letter, by periodically investing in an index fund, the know-nothing investor can actually outperform most investment professionals. This article is for educational purposes only and does not constitute financial or investment advice. Specific fund choices should be made with a qualified financial advisor who understands your individual circumstances.