The Big Casino
In venture capital, the "overhang" is the amount of money put into VC funds by investors which has not yet found a use in the economy. This overhang appears to have exceeded $50 billion in the U.S. alone. In other words, enough people believed that VC would make them rich that they put in more money than the world's startups could productively absorb -- by almost $200 per American.
Hedge funds, working with fungible assets, do not show a literal overhang. However, there is reason to believe that they are overinvested by a similar amount. [A too-large hedge fund cannot implement its profitable strategies without moving the market in the process; thus a portion of its money sits in low-risk passive assets, for which investors continue to pay a substantial management fee.] The investor psychology at work is the same as that in VC.
This appears to demonstrate negative risk aversion -- the VC firms are collectively saying that there are no opportunities worth pursuing, yet they have money thrust upon 'em by the investing public. Individual VC funds then have a choice: they can pocket a management fee (generally 1 to 1.5%) and sit on the cash "waiting for the right opportunity"; or they can gamble on a marginal (i.e., unattractive) startup.
Megan McArdle has noted that saving is a thing of the past, even with large tax incentives; this further shows that a substantial portion of what economists would classify as saving is really closer to gambling [in its motivations, not its economic effects].