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1) few parties can collude to sell or buy at the price they desire. 2) Index fund must balance their portfolio. If stock A goes down and B goes up then they mu

1) few parties can collude to sell or buy at the price they desire. 2) Index fund must balance their portfolio. If stock A goes down and B goes up then they must sell off A and buy more of B to re-balance. At least that has been my understanding. Above two in combination means that you can cause chain reaction which is to be expected because much larger dollars is simply trying to imitate much smaller dollars. So later gets to control the former.

dagw 7y ago
Index fund must balance their portfolio. They must balance to the underlying index, not so that they hold the same value of each stock. If A goes down and B goes up they don't have to do anything unless the change is so dramatic that it causes the people who compile the index to update which stocks are in the index. What can happen is that a stock being added/removed from a major index can cause a large amount of buying/selling by index funds as they are 'forced' to add or remove that stock in their portfolio. However that is a fairly rare occurrence.
OscarCunningham 7y ago
Let's try an example. Suppose company A has 1000000 shares in total, each with market value of $100. Then its market cap is $100 million. Suppose company B has 2000000 shares total, each with market value of $150. Then its market cap is $300 million. Index funds own shares in proportion to the market cap. So an index fund will own shares of B worth three times as much as their shares in A. Lets say we're looking at a fund that owns $6 million of B and $2 million of A. Now suppose A's share price doubles to $200 and B's share price falls to $125. Then the value of the index funds shares will now be $2 million × ($200/$100) = $4 million of A, and $6 million × ($125/$150) = $5 million of B. Meanwhile A's market cap has changed to 1000000 × $200 = $200 million, and B's market cap has changed to 2000000 × $125 = $250 million. Then B's market cap is 1.25 of A's market cap, so the index fund wants to be holding shares in B worth 1.25 the value of its shares in A. It currently has $4 million of A, so it wants to be holding 1.25 × $4 million = $5 million of B, which in fact is the amount that it is already holding.

dagw 7y ago

Index fund must balance their portfolio. They must balance to the underlying index, not so that they hold the same value of each stock. If A goes down and B goes up they don't have to do anything unless the change is so dramatic that it causes the people who compile the index to update which stocks are in the index. What can happen is that a stock being added/removed from a major index can cause a large amount of buying/selling by index funds as they are 'forced' to add or remove that stock in their portfolio. However that is a fairly rare occurrence.

OscarCunningham 7y ago

Let's try an example. Suppose company A has 1000000 shares in total, each with market value of $100. Then its market cap is $100 million. Suppose company B has 2000000 shares total, each with market value of $150. Then its market cap is $300 million. Index funds own shares in proportion to the market cap. So an index fund will own shares of B worth three times as much as their shares in A. Lets say we're looking at a fund that owns $6 million of B and $2 million of A. Now suppose A's share price doubles to $200 and B's share price falls to $125. Then the value of the index funds shares will now be $2 million × ($200/$100) = $4 million of A, and $6 million × ($125/$150) = $5 million of B. Meanwhile A's market cap has changed to 1000000 × $200 = $200 million, and B's market cap has changed to 2000000 × $125 = $250 million. Then B's market cap is 1.25 of A's market cap, so the index fund wants to be holding shares in B worth 1.25 the value of its shares in A. It currently has $4 million of A, so it wants to be holding 1.25 × $4 million = $5 million of B, which in fact is the amount that it is already holding.