An index fund does not own “the index.” An index is a list and a weighting rule. The fund is a pooled company that buys some or all of the securities on that list. You own shares of the fund. Those shares are a claim on the fund’s portfolio, not stock certificates with your name on them.
You cannot invest directly in a market index. The S&P 500, the Russell 2000, and the Wilshire 5000 Total Market Index are examples of lists. Each list measures a basket of securities. The list is a measuring stick. The fund is the thing you can buy.
The fund owns the portfolio. You own the fund.
An index fund is usually a mutual fund or an exchange-traded fund: an SEC-registered investment company that pools money and holds stocks, bonds, or other assets. The combined holdings are the portfolio. Each share is a slice of that portfolio and of the income it produces. The stocks sit in the fund’s name. Your name is on the fund share.
That is why a ticker that “tracks the S&P 500” is not a shortcut that puts 500 certificates in your account. It is a claim on whatever the fund actually holds, minus fees and liabilities. If the fund is a mutual fund, you buy and redeem shares with the fund at the next calculated net asset value. If it is an ETF, you buy and sell shares on an exchange at a market price. The portfolio underneath is still the fund’s.
The fund does not have to hold every name on the list. Some funds buy all of the securities in the index, in roughly the same weights. That is full replication, or close to it. Other funds buy a sample that is meant to behave like the whole list. The SEC’s own bulletin says both approaches are normal. Some funds also use derivatives — options or futures — to help track the objective. Two funds with the same index in the name can hold different baskets and deliver different returns.
The list has a weighting rule
Most broad stock indexes weight by market capitalization: share price times shares outstanding. A larger company is a larger slice of the index, so it is a larger slice of a full-replication fund. A few lists, including the Dow Jones Industrial Average, are price-weighted. The weight then follows the price per share, not the company’s total market value.
The fund is trying to match the list, not outguess it. That is the passive part. An actively managed fund can buy and sell without regard to an index, so long as the trades still fit its objective. An index fund generally cannot step aside when a name in the list falls. It is built to stay pointed at the list.
It will still take the same general risks as the securities in that list. It can also miss the index. Sampling, trading costs, and fees all create tracking error. Even a cheap fund can underperform its index because the index itself has no expenses.
What the share is not
Owning the fund share is not the same as voting the underlying shares in your own name, directing which securities go in, or having FDIC insurance on the portfolio. Mutual funds and ETFs are not bank deposits. The value can go down.
Naming a well-known index is not a recommendation to buy a fund that tracks it. Read the prospectus for what the fund actually holds, how it samples, and what it charges.