...Grantham, who says that “this time it’s different are the four most dangerous words in the English language,” has become a connoisseur of bubbles. His historical study of more than 300 of them shows the same pattern occurring again and again. A bump in sales or some other impressive development causes people to get excited. When they do, the price of that asset class — South Sea company shares, dot-coms — goes up, and human nature and the financial industry conspire to push it higher. People want to hear good news; they tend to be bad with numbers and uncertainty, and to assume that present conditions will persist. In the financial industry, the imperative to minimize career risk produces herd behavior. As John Maynard Keynes, one of Grantham’s heroes, put it, “A sound banker, alas! is not one who foresees danger and avoids it, but one who, when he is ruined, is ruined in a conventional and orthodox way along with his fellows, so that no one can really blame him.” All these factors contribute to a surge of what Keynes called “animal spirits,” which encourages people to convince themselves that this time prices will just rise and rise.