In traditional or active investing, managers will buy cheap stocks to raise their price and then sell them off when they're worth more. But that dynamic is not at play with passive investors. Steve says, if a new stock enters the s and p 500, then passive investors have to buy it and keep it. So a stock can just keep going up or keep falling, without there being as many active investors to come in and push it back. The estimates are that passive investment maybe accounts for 30 % of the us Stock market,. somewhere around there.