There have always been economic booms and busts and, sooner or later, stock markets are bound to crash. The key question for investors is: what, if anything can you do about it? How can you build a portfolio to withstand a crash and minimise its impacts? And how can you avoid being swept along by the euphoria that tends to precede a crash?

ROBIN POWELL has been putting these questions to an expert on bubbles and crashes — JOHN TURNER, Professor of Finance and Financial History at Queen’s University Belfast.