Index funds

What an index fund actually is

An index is just a published list with rules: the S&P 500 is roughly the 500 largest US companies, a total-market index is essentially every listed US company. An index fund is a pooled vehicle that holds those companies in the same proportions the list specifies, and does nothing else.

Everything follows from "does nothing else." There is no analyst team forming opinions, so there is no analyst team to pay. There is no view to act on, so the fund trades only when the list changes or when money flows in and out — which keeps transaction costs and taxable capital-gain distributions low. You are not buying skill. You are buying the average, cheaply, and the average is the thing that has historically produced the returns used throughout this site.

Owning one broad index fund also means owning thousands of businesses at once, so a single company can go to zero without ending your plan.

Why it wins: arithmetic first, evidence second

The case for indexing is not that active managers are stupid. It is arithmetic, and William Sharpe set it out in "The Arithmetic of Active Management" (1991). All the shares in a market are held by somebody. Split those holders into passive and active. The passive ones, by construction, earn the market return before costs. Therefore the remaining dollars — the actively managed ones, in aggregate — must also earn the market return before costs, because the two groups together are the market. Active management costs more to run. So the average actively managed dollar must earn less than the market after costs. This is true before anyone examines a single track record; it would hold in a world where every manager was brilliant.

The empirical work confirms what the algebra requires. S&P Dow Jones Indices publishes the SPIVA U.S. Scorecard, which measures active funds against the benchmarks they claim to beat. Over 10- and 15-year windows it has consistently found the large majority of active US large-cap funds trailing their index, and the longer the window the worse the record. The handful who did beat it are only identifiable afterwards, which is no help when you have to choose today.

What fees actually cost

Fees are the one input you control with certainty. Take $1,000 a month for 30 years at a 7% gross return — $1,219,971 with zero costs — and subtract each fund's expense ratio directly from the return, so a 0.50% fund compounds at 6.50%. That is a simplification, since the fee is charged on assets rather than on return, but it is close.

$1,000 invested at the end of each month for 360 months, compounded monthly at the stated net rate. Expense ratios shown are illustrative of the ranges available, not quotes for specific funds.

A 1% fee does not cost you 1%. It costs roughly 18% of the final balance, because the money the fee removes each year would itself have compounded — the mechanism is spelled out in compound interest. And you pay it whether the fund wins or loses. Run other fee assumptions in the compound interest calculator.

Choosing one

Total market or S&P 500. A total-market fund adds mid- and small-cap companies to the large ones; an S&P 500 fund holds the large caps only. Their returns track each other closely because the biggest companies dominate both by weight. Either is a defensible core holding. Do not own both and imagine you have diversified; you have bought the same companies twice.

Target-date funds. These hold a mix of stock and bond index funds and shift toward bonds as the target year approaches. If you want one fund and no decisions, this is the honest answer, and it is why they are the default in most 401(k) plans. The cost is a higher expense ratio and a glide path someone else chose. What that stock/bond split should be is the subject of asset allocation.

The expense ratio is the number that matters. Two funds tracking the same index hold the same companies; the cheaper one wins by exactly the fee difference. Past performance charts for index funds tell you about the index, not about the fund. Ignore star ratings entirely.

ETF or mutual fund. Usually two share classes of the same portfolio. ETFs trade during the day and often have a lower minimum; mutual funds price once at close and support automatic recurring investment more cleanly, which matters for dollar-cost averaging. In a Roth IRA or 401(k), the tax differences between them are irrelevant.

International allocation is genuinely unsettled. Roughly 40% of global market capitalisation sits outside the US. Holding it insures against a decade of US underperformance, which has happened before. Holding none is a concentrated bet that has been the correct one for the past fifteen years. Serious people land anywhere between 0% and 40%, and nobody knows which will look right in 2050. Pick a number, write down why, and stop revisiting it.

Buying the first one

Start with the account you already have: your 401(k) contains a fund menu, and the cheapest broad index option in it is usually obvious once you sort by expense ratio. Outside work, an IRA or a taxable account gets funded by transfer from your bank.

Then three steps. Move cash into the account — it sits there uninvested until you act, which is the single most common mistake. Place the buy order for the fund you chose. Set the recurring contribution and, if offered, automatic dividend reinvestment, so the position grows without further decisions; reinvested distributions are also why chasing yield through dividend investing adds little to a total-return strategy.

After that, the job is not touching it. One broad, cheap index fund, funded every month for decades, is what the arithmetic above recommends and what most of the plans in investing are built on.

Educational content, not financial, tax, or legal advice. Figures are illustrations based on stated assumptions, not guarantees; markets involve risk, including loss of principal.