How index funds actually work

An index fund is a pooled investment whose holdings are dictated by a published set of rules. The rules — the index methodology — decide which securities are eligible, how they are weighted, and how often the list is refreshed. The fund manager's job is administrative rather than predictive: hold what the rulebook says, in the proportions it says, at the lowest achievable cost.

The index is the product

Investors often describe an index as 'the market', but every index is an editorial choice. A large-cap US index and a total-market US index will diverge in what they exclude. A weighting scheme decides whether the largest companies dominate returns or whether each holding contributes equally. Two funds with near-identical names can hold materially different portfolios because they license different methodologies.

• Eligibility rules: which listings, domiciles and share classes qualify.
• Weighting: market-capitalisation, equal weight, or a factor tilt.
• Rebalancing: how often the fund must trade to stay aligned.
• Buffer rules: how much drift is tolerated before a change is forced.

Replication: full, sampled or synthetic

A fund can hold every constituent (full replication), hold a representative subset chosen to behave like the index (sampling), or gain exposure through a swap contract with a bank (synthetic). Sampling is common in bond indices, where thousands of illiquid issues make full replication impractical. Synthetic replication introduces counterparty risk, which regulated funds mitigate with collateral but do not eliminate.

Tracking difference is the scorecard

The honest measure of an index fund is not its return but how closely that return matches the index after costs. Ongoing charges, trading costs at rebalance dates, cash drag and withholding tax on dividends all pull a fund below its benchmark; securities lending revenue can push it back up. Fund documents publish these figures, and they are the numbers worth comparing.

Where index funds can disappoint

Index investing guarantees market returns minus costs — including in a falling market. A capitalisation-weighted fund becomes more concentrated as its largest holdings rise, which is a feature of the design rather than a fault. And an index that looks broad by name may be narrow by exposure if a handful of sectors dominate its weighting.

None of this makes index funds unsuitable. It makes them a tool with a specification. Reading the specification is the work.