“Do you know what a bell curve is? It’s a curve, shaped like a bell.”

London mayoral candidate Boris Johnson, speaking at the Oxford Union in 1985

As investors we put a lot of weight on the bell curve, perhaps too much weight, and certainly more than we realise. Our investment and risk decisions depend on it. Mr Johnson was describing what statisticians call the “normal distribution,” which frequently recurs in nature. As the diagram shows it is, indeed, shaped like a bell.