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At what point do index funds become unreliable?
A great deal of analysis on this question relies on misunderstandings of the market or noticing trends that happened at the same time but were not caused by each other.  Without knowing your view, I'll just give the basic idea. The amount of active management is self-correcting.  The reason people have moved out of actively managed funds is that the funds have not been performing well.  Their objective is to beat their benchmarks by profiting as they correct mispricing.  They are performing poorly because there is too much money chasing too few mispricings.  That is why the actively managed industry is shrinking.  If it gets small enough, presumably those opportunities will become more abundant and mispricing correction will become more profitable.  Then money will flow back into active funds. Relevant active management may not be what a lay person is thinking of.  At the retail level, we are observing a shift to passive funds, but there is still plenty of money in other places.  For example, pension and endowment funds normally have an objective of beating a market benchmark like the Russell 3000.  As a result they are constantly trying to find opportunities to invest in active management that really can outperform.  They represent a great deal of money and are nothing like the "buy and forget" stereotype we sometimes imagine.  Moreover, hedge funds and propreitary trading shops explicitly and solely try to correct mispricings.  They represent a very, very large bucket of money that is not shrinking.  Active retail mutual funds and individual investors are not as relevant for pricing as we might think.  More trading volume is not necessarily a good thing, nor is it the measure of market quality.  One argument against passive funds is that passive funds don't trade much.  Yet the volume of trading in the markets has risen dramatically over time as a result of technological improvements (algorithmic traders, mostly).  They have out-competed certain market makers who used to make money on inefficiencies of the market.  Is this a good thing or a bad thing?  Well, prices are more efficient now and it appears that these computers are more responsive to price-relevant information than people used to be.  So even if trading volume does decrease, I see no reason to worry that prices will become less efficient.  That's not the direction things have gone, even as passive investing has boomed. Overall, worries about passive investing rely on an assumption that there is not enough interest in and resources for making arbitrage profits to keep prices efficient.  This is highly counterfactual and always will be.  As long as people and institutions want money and have access to the markets, there will be plenty of resources allocated to price correction.