http://en.wikipedia.org/wiki/Economic_bubble
Possible causes
The cause of bubbles is a puzzle to science. It has been variously suggested that bubbles may be rational,[7] intrinsic,[8] and contagious,[9]. To date, there is no widely accepted theory to explain their occurrence.
Puzzlingly, bubbles occur even in highly predictable experimental markets, where uncertainty is eliminated and market participants should be able to calculate the intrinsic value of the assets simply by examining the expected stream of dividends.[3] Nevertheless, bubbles have been observed repeatedly in experimental markets, even with sophisticated participants such as business students, managers, and professional traders. Experimental bubbles have proven robust to a variety of conditions, including short-selling, margin buying, and insider trading.[1]
While it is not clear what causes bubbles, there is evidence to suggest that they are not caused by bounded rationality or assumptions about the irrationality of others, as assumed by greater fool theory. It has also been shown that bubbles appear even when market participants are well-capable of pricing assets correctly.[5] Further, it has been shown that bubbles appear even when speculation is not possible[4] or when over-confidence is absent.[5]
[edit] Social psychology factors
[edit] Greater fool theory
Popular among laymen but not fully confirmed by empirical research,[4][5] greater fool theory portrays bubbles as driven by the behavior of a perennially optimistic market participants (the fools) who buy overvalued assets in anticipation of selling it to other rapacious speculators (the greater fools) at a much higher price. According to this unsupported explanation, the bubbles continue as long as the fools can find greater fools to pay up for the overvalued asset. The bubbles will end only when the greater fool becomes the greatest fool who pays the top price for the overvalued asset and can no longer find another buyer to pay for it at a higher price.
[edit] Greed
A related explanation is that economic bubbles are favored by the greed and irrational exuberance of overly bullish investors. The argument is that investors tend to extrapolate past extraordinary returns on investment of certain assets into the future, causing them to overbid those risky assets in order to attempt to continue to capture those same rates of return. Overbidding on certain assets will at some point result in uneconomic rates of return for investors; only then the asset price deflation will begin. When investors feel that they are no longer well compensated for holding those risky assets, they will start to demand higher rates of return on their investments.
[edit] Herding
Another related explanation used in behavioral finance lies in herd behavior, the fact that investors tend to buy or sell in the direction of the market trend. This is sometimes helped by technical analysis that tries precisely to detect those trends and follow them, which creates a self-fulfilling prophecy.
[edit] Liquidity
Others argue that the cause of bubbles is excessive monetary liquidity in the financial system. However, this explanation cannot be complete, because bubbles appear even in experimental markets that are carefully controlled.[3] According to the explanation, excessive monetary liquidity (easy credit, large disposable incomes) potentially occurs while central banks are implementing expansionary monetary policy (i.e. lowering of interest rates and flushing the financial system with money supply). When interest rates are going down, investors tend to avoid putting their capital into savings accounts. Instead, investors tend to leverage their capital by borrowing from banks and invest the leveraged capital in financial assets such as equities and real estate.
Simply put, economic bubbles often occur when too much money is chasing too few assets, causing both good assets and bad assets to appreciate excessively beyond their fundamentals to an unsustainable level. The bubbles will burst only when the central bank reverses its monetary accommodation policy and soaks up the liquidity in the financial system. The removal of monetary accommodation policy is commonly known as a contractionary monetary policy. When the central bank raises interest rates, investors tend to become risk averse and thus avoid leveraged capital because the costs of borrowing may become too expensive.
[edit] Other possible causes
Some regard bubbles as related to inflation and thus believe that the causes of inflation are also the causes of bubbles. Others take the view that there is a "fundamental value" to an asset, and that bubbles represent a rise over that fundamental value, which must eventually return to that fundamental value. There are chaotic theories of bubbles which assert that bubbles come from particular "critical" states in the market based on the communication of economic factors. Finally, others regard bubbles as necessary consequences of irrationally valuing assets solely based upon their returns in the recent past without resorting to a rigorous analysis based on their underlying "fundamentals".