BUILD a better mousetrap, the saying goes, and the world will beat a path to your door. Find a way to beat the stockmarket and they will construct a high-speed railway. As investors try to achieve this goal, they draw on the work of academics. But in doing so, they are both changing the markets and the way academics understand them.

The idea that financial markets are “efficient” became widespread among academics in the 1960s and 1970s. The hypothesis stated that all information relevant to an asset’s value would instantly be reflected in the price; little point, therefore, in trading on the basis of such data. What would move the price would be future information (news) which, by definition, could not be known in advance. Share prices would follow a “random walk”. Indeed, a book called “A Random Walk Down Wall Street” became a bestseller.

The idea helped inspire the creation of index-trackers—funds that simply buy all the shares in a benchmark like the S&P...Continue reading