Bubbles and Mammoth

Paul Krugman on the reason for momentum in equity prices.

And those instincts can be self-reinforcing, at least for a while. After all, whereas an increase in the number of people acting like Cave Bulls tended to mean fewer mammoths per hunter, an increase in the number of modern bulls tends to produce even bigger capital gains - as long as the run lasts. Any broker can tell you that in the last few months the market has been rising, despite mediocre earnings news, because of fresh purchases by ever more people distraught about having missed out on previous gains and desperate to get in on the action. Sooner or later the supply of such people will run out; then what?