The claim I've often heard repeated [1] is that there are investors who leave their properties unoccupied rather than renting them out, satisfied to bank the returns from price appreciation without the hassle of tenants. So-called "buy-to-leave" properties. For example, one report found just 19 out of 80 apartments in an expensive development were occupied [2].

This has the pernicious effect that it undermines people's belief that increasing supply/housing density will lower prices. After all, why fight for something you don't think will benefit you?

Instead, people who believe such things prefer policies of government intervention, like social housing, taxes on empty properties and suchlike.

[1] https://pbs.twimg.com/media/DXSUbv3X4AAO-dK.jpg [2] http://uk.businessinsider.com/property-partner-20000-ghost-h...