Australia has a divorce rate of around 40%. Yet ask any newlyweds what the odds are that their own marriage will end in separation and their estimate is usually just shy of zero. ‘Remarriage is the triumph of hope over experience’, as author Samuel Johnson put it. We all like to think of ourselves as perfect realists, but the truth is that we often suffer from what psychologists call optimism bias: the tendency to overestimate the probability that good things will happen to us and underestimate the risk of adverse events.