Scenario planners, concerned in part that people may imagine the future too narrowly, encourage the creation of alternative possible future scenarios as a tool. Philip Tetlock, who has written a book about experts’ ability to forecast, did an experiment:
Before the exercise, people’s probabilities at least added to about 100%. The exercise led participants to violate this elementary logical principle: after thinking about different scenarios, every outcome seemed more likely. For the Canadian problem, the average expert gave all the possibilities a 158% chance of happening. The error, in essence, was that merely imagining possibilities, however outlandish, made people take them seriously.
As the comment in which I found this concludes, ‘it’s not that hard to make people suffer from excessive open-mindedness’.