Index Fund Investing Explained
What index funds track, why low costs matter so much over time, and how they fit into a long-term plan.
⏱️ ~7 min read
Key Takeaways
- An index fund aims to match the performance of a specific market index, not beat it
- Because they're passively managed, index funds typically have very low expense ratios
- Over long periods, low costs and broad diversification have made index funds difficult for many active strategies to consistently outperform
- Index investing doesn't mean 'no risk' — an index fund still falls when its underlying index falls
The core idea: match the market, don't try to beat it
An index fund is a mutual fund or ETF designed to replicate the performance of a specific market index — such as the S&P 500, a total bond market index, or an international stock index — by holding the same securities in the same (or very similar) proportions as that index.
This is described as 'passive' management, in contrast to 'active' management, where a fund manager makes ongoing decisions about which securities to buy, sell, or weight differently than the index, aiming to outperform it.
Why costs are so low
Because an index fund's holdings are determined mechanically by the index it tracks — rather than by a team of analysts doing research and making active decisions — the fund's operating costs are typically much lower. This is reflected in a lower expense ratio, often a small fraction of a percent for broad index funds.
Lower costs mean a larger share of the index's return is passed through to investors, since fees are deducted directly from fund returns regardless of whether the fund goes up or down.
The 'difficult to beat' track record
A substantial body of research and long-running performance comparisons has shown that, over long time periods, a majority of actively managed funds in many categories have underperformed their relevant benchmark index after fees. This doesn't mean no active manager ever outperforms — some do, in some periods — but consistently identifying which ones will do so in advance, ahead of time, has proven very difficult, even for professional investors.
This track record is a major reason index funds have grown enormously in popularity — they offer a way to capture a market's overall return, broad diversification, and low costs, without needing to predict which individual securities or managers will outperform.
What index investing doesn't protect you from
An index fund tracking the S&P 500 will fall when the S&P 500 falls — including during bear markets. Index investing doesn't eliminate market risk; it simply means your returns will closely mirror the index's returns (minus the small fee), for better or worse.
It's also possible to be under-diversified while using only index funds, if those funds overlap heavily or are concentrated in a single market or sector. A 'total market' index fund is broader than a narrower index fund focused on one sector or country.
Example: Tracking error
An index might return 10.00% over a year.
An index fund tracking it, after its 0.05% expense ratio and minor trading costs, might return approximately 9.94% — very close to, but not exactly, the index's return. This small gap is called 'tracking error,' and is generally larger for funds with higher costs or that hold a sample of the index rather than every single security.
How index funds fit a long-term plan
Many long-term investors use one or more broad index funds — covering domestic stocks, international stocks, and bonds — as the 'core' of their portfolio, in proportions that match their time horizon and risk tolerance (see 'How to Build Your First Portfolio'). Because of their low cost and broad diversification, they're often used as a simple, durable foundation that doesn't require ongoing security selection decisions.
Frequently Asked Questions
Which index should a beginner consider?+
Broad, diversified indexes — such as a total US stock market index or the S&P 500 for US large companies, often paired with a total bond market index and/or international stock index — are commonly used as core building blocks, though the right mix depends on individual goals.
Do index funds ever change what they hold?+
Yes — when the underlying index adds, removes, or reweights its constituent securities (which index providers do periodically based on their rules), the index fund adjusts its holdings to match, generally without any action needed from the investor.
Is index investing 'set and forget' forever?+
Index funds reduce the need for ongoing security selection, but investors may still periodically rebalance their overall allocation across different index funds (e.g., stocks vs. bonds) as their goals, time horizon, or risk tolerance change.