What Is an ETF and How Does It Work?

A basket of securities that trades like a single stock

An ETF (Exchange-Traded Fund) is a fund that holds a collection of assets — stocks, bonds, commodities, or a mix — and issues shares that trade on a stock exchange throughout the day, just like Apple or Tesla. When you buy one share of SPY (the S&P 500 ETF), you effectively own a proportional slice of all 500 companies in the S&P 500 index. You receive the diversification of 500 companies with a single transaction at the cost of a few dollars in annual fees.

The ETF structure was invented in Canada in 1990 and arrived in the US in 1993 with the launch of SPY — now the largest ETF in the world by assets ($550+ billion). The innovation was combining the diversification of a mutual fund with the tradability of a stock. Before ETFs, building a diversified portfolio meant either buying hundreds of individual stocks (expensive, complex) or using a mutual fund (priced once daily, high fees, tax-inefficient). ETFs provided both diversification and intraday liquidity at a fraction of the cost.

Net Asset Value — the fair value anchor

An ETF's true value is its Net Asset Value (NAV) — the total market value of all its underlying holdings divided by the number of shares outstanding. SPY holds all 500 S&P 500 stocks in proportion to their market cap. If those 500 stocks have a combined value of $550 billion and SPY has 1 billion shares outstanding, NAV = $550. If SPY's market price is $549.80, it trades at a 0.036% discount to NAV.

In practice, ETF prices stay within fractions of a percent of NAV because of a powerful self-correcting mechanism involving authorised participants (APs). These are large institutional firms (typically investment banks) that have the unique right to create or redeem large blocks of ETF shares — called creation units, typically 50,000 shares — directly with the fund issuer. If SPY trades below NAV, an AP buys SPY shares on the market (below fair value), then redeems them with the fund issuer in exchange for the underlying stocks (worth more). They immediately sell the stocks for a risk-free profit. This arbitrage buying pushes SPY's price back up toward NAV.

Why active managers fail over time

S&P's SPIVA report is the most comprehensive scorecard of active vs passive fund performance. The 2023 edition found that over 20 years, 92% of actively managed US large-cap funds underperformed the S&P 500. Over 15 years: 90%. Over 10 years: 88%. These numbers are not idiosyncratic to the US — the story repeats across virtually every asset class, geography, and time period studied. Only a small minority of active managers beat their benchmark net of fees over long periods, and identifying those managers in advance — before the outperformance, not after — is essentially impossible.

Why do most active managers fail? Three interconnected reasons. First, fees: active funds charge 0.5–1.5% annually vs 0.03–0.20% for index ETFs. This creates a compounding headwind that managers must overcome before generating any net benefit for investors. Second, market efficiency: in liquid markets like US large-cap equities, prices reflect available information very rapidly. The analysts at active funds are competing against thousands of other analysts with similar information access — finding genuine mispricings consistently is extraordinarily difficult. Third, survivorship bias: SPIVA accounts for this, but the broader industry reporting does not. Funds that underperform badly are often closed and merged into better performers, removing their poor track record from the conversation.

The Buffett Bet — a decisive 10-year experiment

In 2007, Warren Buffett publicly bet $1 million that a simple S&P 500 index fund would outperform any selection of at least five actively managed hedge funds over 10 years. Protégé Partners, a respected fund-of-funds manager, accepted. They selected five hedge funds of funds representing approximately 200 individual hedge funds. Result by January 2018: Buffett's Vanguard S&P 500 Admiral Fund: +125.8% cumulative. Protégé's hedge funds: +36.3% cumulative. The 90-percentage-point gap was not because the underlying hedge fund managers were incompetent — it was largely because the double-layer fee structure (hedge fund fees + fund-of-funds fees) compounded devastatingly against gross returns. Buffett donated the winnings to Girls Inc. of Omaha. The experiment became one of the most widely cited arguments for passive indexing.

The in-kind redemption tax advantage

One of the most practically important ETF advantages in taxable brokerage accounts is tax efficiency. When investors sell a mutual fund, the fund must redeem their shares by selling underlying holdings to raise cash — potentially triggering capital gains that are distributed to all remaining fund investors, even those who did not sell. In 2021, several large actively managed funds distributed capital gains of 10–20% of NAV as taxable distributions to investors — hitting them with tax bills for gains they personally never chose to realise.

ETFs avoid this through in-kind creation/redemption. When an authorised participant redeems ETF shares, they receive the underlying stocks directly — no stocks are sold, no cash changes hands, and no capital gains event is triggered inside the fund. This means ETF investors in taxable accounts only incur capital gains tax when they personally sell their ETF shares — not from other investors' redemption activity. Over a 30-year holding period in a taxable account, this structural tax efficiency advantage can be worth 0.3–1% per year in after-tax returns — often exceeding the expense ratio advantage over active funds.

What to look for when choosing your first ETF