What is an index fund

An index fund is a fund that tries to match the return of a particular market index rather than beat it.

The SEC's own definition, from investor.gov, is more precise and worth reading closely. A passive fund is a mutual fund, exchange-traded fund or unit investment trust that follows an investment strategy designed to achieve approximately the same return as a particular index, before fees. And: passively managed mutual funds, ETFs and UITs are also called index funds.

Two things fall out of that immediately.

Index fund is a strategy, not a product type. It describes what the manager is trying to do. The wrapper can be a mutual fund, an ETF or a UIT, and the choice of wrapper changes how you buy it, how it is taxed and how it trades. We have covered the real difference between an ETF and a mutual fund separately.

The definition contains the word approximately, and the phrase before fees. Those are not hedging. They are the whole story.

Why it cannot match the index exactly

The SEC puts this more plainly than most fund marketing does: market indexes themselves have no expenses, so even a passively managed index fund can underperform its index because of fees and taxes.

An index is a calculation. It does not pay a manager, a custodian, a transfer agent or a commission. A fund does all of those things. The gap between the index and the fund is therefore built in, and the fund's job is to keep it small rather than to close it.

There is a second source of drift. The SEC notes a passive strategy can be implemented either by buying every security in the index or by holding a representative sampling. Sampling is normal and often sensible, particularly for indexes with thousands of illiquid constituents, but a sample is not the index, and it tracks it imperfectly by construction.

What the expense ratio actually covers

Investor.gov defines the expense ratio as a fund's total annual operating expenses, including management fees, distribution or 12b-1 fees and other expenses, expressed as a percentage of average net assets.

That is a broader figure than a management fee alone, which is why comparing management fees across funds can mislead. We have written on what counts as a cheap expense ratio and what the number does and does not include.

The expense ratio is also not the only cost. Trading costs inside the fund, bid-ask spreads when you buy an ETF, and any brokerage commission sit outside it.

Why index funds are usually cheaper

The SEC's explanation is mechanical rather than ideological. Passive management usually translates into less trading of the fund's portfolio, which means lower transaction costs. Less trading also tends to realise fewer capital gains, which the SEC describes as more favourable income tax consequences. And overseeing a rules-based strategy requires less of the research apparatus that active management funds.

An older SEC study quantified part of this: it found that in 1999, other things held equal, the operating expense ratio of an index fund was 45 basis points lower than an equivalent non-index fund. That figure is from 1999 and should be read as historical rather than current, because fee competition has moved a great deal since.

Note the word usually in the SEC's framing. It is not a guarantee. An expensive index fund is entirely possible, and the strategy label does not cap the fee.

Not every fund calling itself an index fund is simple

The SEC has a separate bulletin on smart beta, quant funds and other non-traditional index funds. Its position is that these still count as passive, because the adviser seeks to track an index rather than using independent judgement to pick investments.

The subtlety is that the index itself can be anything. A bespoke index constructed around a particular factor or screen is still an index, and a fund tracking it is still technically passive, while behaving very differently from a fund tracking a broad market measure. The label tells you about the manager's discretion, not about what you own.

How to read one

Three questions, all answerable from the documents.

Which index, and how is it constructed? A broad market index and a custom factor index are different exposures wearing the same word.

Full replication or sampling? Both are legitimate; they drift differently.

What is the total expense ratio, and what sits outside it? The ratio covers operating expenses. Spreads, commissions and internal trading costs do not appear in it.

This is explanatory information about how a product category works, not advice about whether any fund suits your circumstances. That question belongs with a licensed adviser who knows them.