James finds two funds that follow the same S&P 500 index. One is an ETF, and the other is an index mutual fund. Also, their holdings are almost identical, both charge very low fees, and their prices move together. So does the choice matter at all?
It can, although usually not for the reason beginners expect. In fact, the ETF is not automatically more profitable, and the mutual fund is not automatically safer. Instead, the differences show up in how you buy, how you automate, what you pay in total and, in some accounts, how much tax you owe. So in this lesson, we compare real funds, test the choice against 20 years of real market data and show which difference matters in your situation.
Same index, different package
First of all, an index fund is a strategy, not a structure. In fact, as Investor.gov explains, a company can set up an index fund either as a mutual fund or as an ETF. Either way, the fund pools investors’ money and tries to copy the same index. Therefore, the difference is the package around the portfolio, as we explain in Stocks, Bonds, Mutual Funds and ETFs.
For example, here is how four real S&P 500 funds compare. Although all four follow the same index, they come in different packages:
| Fund | Structure | Expense ratio | Minimum to start |
|---|---|---|---|
| Fidelity 500 Index Fund (FXAIX) | Index mutual fund | 0.015% | $0 |
| Schwab S&P 500 Index Fund (SWPPX) | Also an index mutual fund | 0.02% | $0 |
| Vanguard 500 Index Admiral (VFIAX) | Index mutual fund too | 0.04% | $3,000 |
| Vanguard S&P 500 ETF (VOO) | ETF | 0.03% | One share, or even less with fractional shares |
Fees and minimums come from the fund companies’ published information, although they can change, so always check the latest prospectus.
In other words, the cheapest fund on this list is a mutual fund, not an ETF. So the old idea that ETFs are always cheaper is no longer true for the biggest index funds.
Difference 1: how and when you buy
An ETF trades on the stock exchange during the day, like a single stock. So you see a live price, and you buy at whatever price you get when your order fills. A traditional mutual fund, by contrast, trades once a day: if you place an order before the cutoff, you get that day’s closing price, called the net asset value or NAV. Indeed, the SEC’s comparison of the two describes this difference in detail.
What the spread means
ETF trades also involve a bid-ask spread: the small gap between the price buyers offer and the price sellers ask. For example, imagine an ETF with a bid of $49.98 and an ask of $50.02. So if James buys ten shares at $50.02, he pays $500.20. Then, if he sold immediately at $49.98, he would get $499.80. In short, that 40-cent gap is the spread. For very popular ETFs like VOO, it is usually tiny, but for small or niche ETFs it can be larger.
A mutual fund has no spread, because everyone gets the same end-of-day price. However, that also means James cannot control the exact price he pays. Still, neither approach is better over the long run; they simply work differently.
Difference 2: automatic investing
Index mutual funds have traditionally been the easiest way to invest an exact amount automatically, for example $200 every payday, because they sell fractional shares. That is also why 401(k) plans usually offer mutual funds rather than ETFs.
Today, however, many brokers also let you buy fractional ETF shares and set up recurring purchases in dollars. As a result, the answer now depends more on your broker than on the fund type. Before choosing, therefore, check four things on the platform you actually use:
- Recurring purchases: first, can you schedule an exact dollar amount?
- Fractional shares: also, can you buy part of an ETF share?
- Dividend reinvestment: then, does the broker reinvest dividends automatically?
- Minimums and fees: finally, does either fund have a minimum or a transaction fee there?
Same index, different fees: 20 real years
If structure matters so little, what does matter? Mainly the fee. For example, the chart below runs three funds that follow the same S&P 500 index through the real yearly returns from 2006 to 2025. In fact, the only difference between them is what they charge.
How to use it: first, enter a starting amount and a monthly addition. Then, if you want, change the three fees, for example to the fees of funds you are considering. As a result, the chart and the three boxes show what each fund would have grown to.
Notice, first, that the green dashed line and the blue line almost sit on top of each other: a low-cost ETF and a low-cost mutual fund ended in nearly the same place. Meanwhile, the red line falls further behind every year. So the real lesson is simple: compare the fee first, and the structure second. Also, we explain how fees build up over time in Expense Ratios Explained.
Difference 3: taxes in a taxable account
In addition, both types of fund can pay out dividends and, sometimes, capital gains. For instance, in a regular brokerage account, those payouts are taxable in the year you receive them, even if you reinvest them. ETFs, however, can often avoid paying out capital gains, thanks to the way they create and redeem shares, which gives them a structural tax advantage.
Still, this matters mainly in a taxable account. Inside a 401(k) or an IRA, payouts are not taxed each year, so the tax difference largely disappears, as the SEC notes. Also, we explain the different account types in 401(k), IRA and Brokerage Accounts. Moreover, the advantage is not a promise: some ETFs do pay out gains, while many large index mutual funds rarely do.
Difference 4: total cost, not just the expense ratio
The expense ratio is the main cost, but it is not the only one. Indeed, the SEC’s bulletin on mutual fund and ETF fees lists other possible costs, such as trading commissions, spreads, sales loads and account fees. For example, a broker might sell its own mutual funds for free but charge a fee for another company’s fund, or the other way around.
So the real comparison is the total cost on the platform you use. “Commission-free” does not make an ETF cost-free, because the spread and the expense ratio still apply. Likewise, “no-load” does not make a mutual fund cost-free.
MONEY TRAP
A live price feels like something to manage
An ETF shows a price that changes every second while the market is open, with buy and sell buttons right next to it. As a result, it can feel like an investment that needs your attention. A mutual fund, by contrast, shows one price a day, so there is much less to react to.
In fact, psychologist Ellen Langer described this effect in a classic study in the Journal of Personality and Social Psychology, The Illusion of Control. She found that people feel more in control of chance outcomes when they can act, choose or watch closely, even when their actions change nothing. Likewise, research by Brad Barber and Terrance Odean found that the households that traded most earned clearly less.
So if you choose an ETF, write one simple rule before buying: you will only change it if its index, cost or role in your plan changes, not because its price moved today.
Which one fits your situation?
There is no universal winner, because people use funds in different ways. Instead, here is where each difference usually matters most:
| Your situation | Key difference | What to check |
|---|---|---|
| Investing every payday | Automation and fractional shares | So check whether your broker supports both |
| Investing through a 401(k) | The plan’s fund list and fees | Simply, which funds the plan offers |
| A taxable brokerage account | Tax efficiency | Also each fund’s history of capital gains payouts |
| Control over the exact price | Trading during the day | In particular, the spread and your order type |
| Fewer decisions | A simple routine | Then, whether live prices tempt you to trade |
General guide for education, so it is not a recommendation of any specific fund.
For example, if James invests $200 a month in an IRA at a broker that automates both types for free, the difference between FXAIX and VOO is tiny. On the other hand, if he invests a lump sum in a taxable account, an ETF’s tax efficiency may tip the balance. Meanwhile, if he invests through a 401(k), his plan decides for him. Alternatively, if he prefers not to choose at all, a target-date fund can do the mixing for him.
Common questions
Is an ETF better than an index mutual fund?
Not automatically. In fact, for the same index and similar fees, both usually give nearly the same long-term result. Therefore, the better choice depends on your account, your broker and how you like to invest.
Can I hold both?
Yes. For example, many people use an index mutual fund in their 401(k) and an ETF in an IRA or brokerage account. Just check that they do not overlap more than you intend, since two S&P 500 funds hold the same companies.
Do ETFs pay dividends?
Yes. In fact, stock ETFs and index mutual funds both pass on the dividends of the companies they hold, usually every quarter. Also, most brokers let you reinvest them automatically.
ABOUT THE AUTHOR
Lorena founded Money Behaves. She is an economist and a psychology student who focuses on how people make money decisions: how they judge risk, react to percentages and read costs, and what happens when financial information gets complicated. She is not a licensed financial advisor, so every lesson relies on official U.S. sources and published research. See how we write and check our lessons.
INVESTING COURSE · COMPLETE
You now know what you own, where to hold it and what it costs.
In short, that covers the core of long-term investing. So next, protect what you are building with the right insurance, or go back and choose your account type: