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CBA is the largest stock in Australia therefore it gets the most passive index/ETF inflows into it due to the ETFs mostly being market cap weighted. This creates a circular effect.
The CBA price goes up due to passive ETFs buying because its the biggest stock, this then pushes the share price even higher which then pushes up its index weighting which then causes the index funds to buy even more.
This will continue until it gets so overpriced that all the non passive investors start selling to take profits and overwhelm the passive buying pushing the price down.