Defining Asset Bubbles

Here is an interesting interview with Jeremy Grantham of investment firm GMO, discussing asset bubbles, what they are, and how they should affect investment decisions.

Economist Edward Harrison at Naked Capitalism adds some analysis and explanation in his post “Jeremy Grantham on Bubbles.” Harrison’s “informal working definition of a bubble is “a price rise that is at least two standard deviations above trend.” He says that

Above two-standard deviations the psychology of prior price movements starts to dominate price activity and a bubble forms in which power law characteristics come into play.

In “power law” relationships, a quantity varies more than you should expect. It kind of refers to the “tall head” part of a distribution graph, the opposite of the “long tail.”

AB — 21 April 2010