In late 2007, the Shanghai stock market was going through a boom. A Professor of Economics was visiting China and there he learnt that everyone, everyone was expecting the stock market to crash after the Olympics in September 2008. In other words, they assumed the market to keep rising like a Diwali rocket till then, and then fall dramatically.
The professor was amused that no one had reasoned backwards from there. Everyone expected the market to fall in September. So, what would happen in August? Wouldn’t they think, “Hey, I don’t want to get too greedy. I know that the index will rise another month, but what if it suddenly starts falling and I am caught short? I had better sell right now and book my profits.”
Now, if many people think so, prices will fall, not in September, but in August. But if everyone knows that everyone else is thinking that way, why would they wait till August? Won’t they start selling in July? And so on, by induction, if people expect the market to tank in September, prices should fall right now.
This is what traditional economics suggests. Behavioural economics argues that the professor’s analysis is incomplete. Obviously, not everyone will decide to sell in August, but some people will. What will happen in the market is the classic battle between greed and fear. Some people, overcome by greed, will hold on in the hope of making even more gains, while some people, overcome with fear, will sell. At some point, fear will overcome greed and the market will crash.