On 8 November 2016 – US election night – the betting odds of Donald Trump winning the presidency had narrowed to five to one. By the following morning, he'd managed to pull off a feat many thought impossible, but that didn't stop the I-told-you-sos – friends, family members and even online soothsayers – who claimed to have known all along that Trump would triumph despite such bad odds.
To a lesser extent it happened in November 2008 too, when Barack Obama won the election. After accepting the Democratic nomination three months earlier, his chance of victory hovered around 60%. When he won, you may well have been one of those who ‘knew’ that was going to be the outcome, and imagined that his odds of winning had been much, much higher. But just because he won didn’t change his odds of winning prior to his victory.
This is hindsight bias – a phenomenon in which we revise probabilities after the fact or exaggerate the extent to which past events could have been predicted beforehand. Politics doesn’t have a monopoly on this: we’re guilty of hindsight bias when we talk about the weather (there’s only a 20% chance of rain, but you say it’s going to rain and it does, and suddenly you have better forecasting ability than the experts); it happens at sporting events, in court rooms, in medical decisions and in business.