This is exactly what happened during the stock market crash of 1929, and regulations were put into place in the 1930s to prevent excessive margin leverage that might result in liquidity problems at brokerages (and in the banks that lend to them). These regulations have been in place since then and probably mitigated the effects of the dot com crash in 2000. See http://en.wikipedia.org/wiki/Regulation_T as a good starting point for research.
Interestingly, I read somewhere that there were similar regulations regarding residential mortgage loans that were repealed during the 1980s, does anybody have a reference to this? I think that requiring a 20% equity/debt ration when originating or refinancing a mortgage loan probably would have made the 2008 real-estate crash look a lot more like the dot com bust and would have saved a lot of economic pain.
Interestingly, I read somewhere that there were similar regulations regarding residential mortgage loans that were repealed during the 1980s, does anybody have a reference to this? I think that requiring a 20% equity/debt ration when originating or refinancing a mortgage loan probably would have made the 2008 real-estate crash look a lot more like the dot com bust and would have saved a lot of economic pain.