Does an index fund comprise of stocks that mimic the index? So when the index is altered to include new players/exclude old ones, the fund also adjusts? But how can this be, since index funds are not actively managed?

And if the index fund just mimics the index, why bother investing in the fund? Why not figure out the % composition of the index and invest in the participating securities directly?

  • The benefit is that you will have investments that (probably) grow over time. :-)
    – poolie
    Commented Apr 18, 2011 at 23:43
  • Investing in the securities directly is laborious when considering large index funds that hold 500 or 3600 titles.
    – Pertinax
    Commented Sep 26, 2018 at 14:52

5 Answers 5


Index funds may invest either in index components directly or in other instruments (like ETFs, index options, futures, etc.) which are highly correlated with the index. The specific fund prospectus or description on any decent financial site should contain these details.

Index funds are not actively managed, but that does not mean they aren't managed at all - if index changes and the fund includes specific stock, they would adjust the fund content. Of course, the downside of it is that selling off large amounts of certain stock (on its low point, since it's being excluded presumably because of its decline) and buying large amount of different stock (on its raising point) may have certain costs, which would cause the fund lag behind the index. Usually the difference is not overly large, but it exists.

Investing in the index contents directly involves more transactions - which the fund distributes between members, so it doesn't usually buy individually for each member but manages the portfolio in big chunks, which saves costs. Of course, the downside is that it can lag behind the index if it's volatile.

Also, in order to buy specific shares, you will have to shell out for a number of whole share prices - which for a big index may be a substantial sum and won't allow you much flexibility (like "I want to withdraw half of my investment in S&P 500") since you can't usually own 1/10 of a share. With index funds, the entry price is usually quite low and increments in which you can add or withdraw funds are low too.


Why not figure out the % composition of the index and invest in the participating securities directly?

This isn't really practical. Two indices I use follow the Russell 2000 and the S&P 500

Those two indices represent 2500 stocks. A $4 brokerage commission per trade would mean that it would cost me $10,000 in transaction fees to buy a position in 2500 stocks. Not to mention, I don't want to track 2500 investments.

Index funds provide inexpensive diversity.


Index funds are good for diversifying risk. For people who don't have a large sum of money to invest, holding all the different types of stocks in the index is both very expensive and not practical because you incur too many transaction costs. For an index funds, the main advantages are that costs are pooled, and investors can invest a smaller amount that they would if they bought all the different stocks individually. Naturally, if you wanted to figure out the percentage composition of the index and invest directly it would be possible, albeit tedious.


Beatrice does a good job of summarizing things. Tracking the index yourself is expensive (transaction costs) and tedious (number of transactions, keeping up with the changes, etc.) One of the points of using an index fund is to reduce your workload. Diversification is another point, though that depends on the indexes that you decide to use. That said, even with a relatively narrow index you diversify in that segment of the market.

A point I'd like to add is that the management which occurs for an index fund is not exactly "active." The decisions on which stocks to select are already made by the maintainers of the index. Thus, the only management that has to occur involves the trades required to mimic the index.

  • 1
    As such the management fees are also much lower. So you may get some win there.
    – Zachary K
    Commented Apr 18, 2011 at 6:32

when the index is altered to include new players/exclude old ones, the fund also adjusts

The largest and (I would say) most important index funds are whole-market funds, like "all-world-ex-US", or VT "Total World Stock", or "All Japan". (And similarly for bonds, REITS, etc.) So companies don't leave or enter these indexes very often, and when they do (by an initial offering or bankruptcy) it is often at a pretty small value.

Some older indices like the DJIA are a bit more arbitrary but these are generally not things that index funds would try to match.

More narrow sector or country indices can have more of this effect, and I believe some investors have made a living from index arbitrage. However well run index funds don't need to just blindly play along with this.

You need to remember that an index fund doesn't need to hold precisely every company in the index, they just need to sample such that they will perform very similarly to the index. The 500th-largest company in the S&P 500 is not likely to have all that much of an effect on the overall performance of the index, and it's likely to be fairly correlated to other companies in similar sectors, which are also covered by the index. So if there is a bit of churn around the bottom of the index, it doesn't necessarily mean the fund needs to be buying and selling on each transition. If I recall correctly it's been shown that holding about 250 stocks gives you a very good match with the entire US stock market.

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