I think the idea that there's currently an 'index tracker bubble' is fascinating: i.e. that the total value of the S&P 500 is no longer based on a multiple of aggregate earnings but is instead driven by finding the price that needs to be paid to achieve the necessary 'churn' for tracker funds to fulfil their mandates as they allocate the funds that their subscribers place with them each month. This creates a scarcity premium that drives up prices and attracts more subcribers. This has a ratchet effect that never previously applied and that will probably prevail until something goes catastrophically wrong.