Index funds are not a clever strategy. They are an ordinary, broadly diversified, low-cost way to participate in the long-term growth of public companies. For most US households, the harder questions are not which fund to buy, but which account to use, how much to contribute, and how to keep going during the years when markets fall.
This guide is written for readers who have heard the term and want a grounded explanation. It does not provide individualized investment advice; for that, talk to a fiduciary financial planner.
What an index fund actually is
An index fund is a pooled investment that aims to replicate the performance of a market index by holding the same securities in roughly the same proportions. A total-market US stock fund holds thousands of US companies. A bond index fund holds a representative slice of the bond market. The fund's job is to track the index, not to beat it.
Index funds come as traditional mutual funds and as exchange-traded funds (ETFs). For most long-term investors the practical differences are small; both can be appropriate.
Cost: the one thing you can control
Returns are uncertain; costs are not. The expense ratio is the annual fee the fund charges, expressed as a percentage of assets. Broad-market US stock index funds today are widely available at expense ratios at or below 0.05%. Bond index funds are similar.
Costs compound the same way returns do. Over a 30-year horizon, an extra 1% in annual fees can reduce a portfolio's end value by roughly 25%. This is the strongest single argument for indexing over actively managed funds with higher fees.
Which account, in what order
For US households, the typical priority order is: contribute enough to a workplace 401(k) to capture the full employer match, then fund a Roth or traditional IRA up to the annual limit, then return to the 401(k) until you hit the contribution cap. Health Savings Accounts (HSAs), for those eligible, sit in this mix as well.
Inside each account, a small number of index funds — sometimes just one target-date fund — is usually sufficient. The decision about which account to use generally matters more than which fund you pick within it.
A simple, defensible portfolio
Many financial planners recommend a 'three-fund' starting point: a total US stock market index fund, a total international stock index fund, and a total US bond market index fund. The proportions depend on your age, your risk tolerance, and your time horizon, but the structure is durable.
A target-date retirement fund accomplishes much the same thing inside a single product, with the asset mix shifting over time. Both approaches are reasonable; the worst portfolio is usually the one you tinker with too often.
The hard part is sitting still
The well-known performance penalty of individual investors versus the funds they hold comes mostly from buying after gains and selling after losses. Index investing reduces this penalty only if you actually stay invested through downturns. Automating contributions removes the moment-by-moment decision.
If you find yourself unable to stay invested during a market decline, that is a signal to revisit your allocation — generally by holding more bonds — rather than to abandon the strategy.