The weather in Chicago was normal for the time of year in March 1952, when Harry Markowitz, an American economist and a native of the Windy City, published an article that put volatility at the heart of a clutch of ideas that came to be known as modern portfolio theory.

Six decades on, and in the wake of a financial crisis born in large part from the proliferation of models based on Markowitz's ideas, investors may be forgiven for wishing the weather had been life-threateningly freezing that month, instead of the usual mild zero degrees celsius. Perhaps the Nobel prize winner would have thought twice before defining risk as volatility.