One of the causes of this is the 1031 tax exchanges (allows a person/company to roll over a gain on real estate into a "like-kind" asset, again: real estate). I have a friend that has rolled over a few houses and duplexes into ever more valuable property (he's done well, but he's levered 3-4x). I've often asked him, "when will you cash out?" His answer, "I can't, I don't want to pay the capital gains tax." "Are you worried about a crash?" "No, because even if rents dip 20% I can make the mortgage payments and repairs and wait for the market to recover." I don't know how much this has driven real estate speculation in my area (Denver, CO), but if there is a semi-serious market turn, enough to cross some sort of pain threshold for speculators, there's got to be some forced selling.