Why do people have such a hard time computing probabilities?
Well you know, there is a cognitive difficulty in thinking about probability in that you have to maintain several possibilities in the mind at once. And this is really something that turns out to be quite difficult. Normally we have a story and its a single story – a single story with a theme. Thinking probabilistically in a proper way is extremely difficult for people. People do have a sense of propensity. That is, that a system has a tendency to do something or other, but that is different than thinking about probability where you do have to think of two possibilities and weigh their relative chances of happening.