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The planning fallacy is an intuitively obvious and scientifically well-measured tendency of people to assume projects will take less time than they do and that the outcome will be better than is justified by past data or experience. For instance, newlyweds almost universally expect their marriages to last a lifetime, when in fact less than half of marriages actually do.
The planning fallacy has been studied by cognitive psychologists who have found evidence for, and strongly suspect that the fallacy is universal across the human species. That people’s predictions are optimistically biased has been called the most robust finding in the psychology of prediction.
There are some exceptions to the planning fallacy. One is that people seem to actually accurately estimate for certain important and personally relevant events such as unwanted pregnancy. It is also well known that people overestimate the probability of rare, highly negative events happening to or around them, such as overestimating death via plane crash while underestimating death via auto accident. These are exceptions to the planning fallacy.
The planning fallacy is likely a special case of overoptimism in general. People have been observed to overestimate the likelihood of good things happening to them. Between 85% and 90% of people feel that their own futures will be better — in terms of health and other things they value — than that of an average peer. In cognitive psychology, there is even a phenomenon called “realistic pessimism,” where very pessimistic people are found to have more accurate predictions in the domain of task prediction than people of optimistic or average dispositions.
It has also been found that people are selectively overoptimistic when the predictions they are making are not personally relevant, and somewhat less so when they are. People have been found to take their skills into account when predicting whether or not they can accomplish something within a given time frame. Optimistic bias can even correlate with improved performance. So the threat of irrational optimism is not as serious as it appears at first sight.