“Once we know the outcome of something, that knowledge skews our perception of what we thought before we knew the outcome: that’s hindsight bias. Baruch Fischhoff was the first to document the phenomenon in a set of elegant experiments. One had people estimate the likelihood of major world events at the time of Fischhoff’s research—Will Nixon personally meet with Mao?—then recall their estimate after the event did or did not happen. Knowing the outcome consistently slanted the estimate, even when people tried not to let it sway their judgment. The effect can be subtle, but it can also be quite big. In 1988, when the Soviet Union was implementing major reforms that had people wondering about its future, I asked experts to estimate how likely it was that the Communist Party would lose its monopoly on power in the Soviet Union in the next five years. In 1991 the world watched in shock as the Soviet Union disintegrated. So in 1992–93 I returned to the experts, reminded them of the question in 1988, and asked them to recall their estimates. On average, the experts recalled a number 31 percentage points higher than the correct figure. So an expert who thought there was only a 10% chance might remember herself thinking there was a 40% or 50% chance. There was even a case in which an expert who pegged the probability at 20% recalled it as 70%—which illustrates why hindsight bias is sometimes known as the “I knew it all along” effect.”