Explanations
Kahneman and Tversky's original explanation for the fallacy was that planners focus on the most optimistic scenario for the task, rather than using their full experience of how much time similar tasks require. One explanation offered by Roger Buehler and colleagues is wishful thinking; in other words, people think tasks will be finished quickly and easily because that is what they want to be the case. In a different paper, Buehler and colleagues suggest an explanation in terms of the self-serving bias in how people interpret their past performance. By taking credit for tasks that went well but blaming delays on outside influences, people can discount past evidence of how long a task should take. One experiment found that when people made their predictions anonymously, they do not show the optimistic bias. This suggests that the people make optimistic estimates so as to create a favorable impression with others.
Some have attempted to explain the planning fallacy in terms of impression management theory.
One explanation, focalism, may account for the mental discounting of off-project risks. People formulating the plan may eliminate factors they perceive to lie outside the specifics of the project. Additionally, they may discount multiple improbable high-impact risks because each one is so unlikely to happen.
Planners tend to focus on the project and underestimate time for sickness, vacation, meetings, and other "overhead" tasks. Planners also tend not to plan projects to a detail level that allows estimation of individual tasks, like placing one brick in one wall; this enhances optimism bias and prohibits use of actual metrics, like timing the placing of an average brick and multiplying by the number of bricks. Complex projects that lack immutable goals are also subject to mission creep, scope creep, and featuritis. As described by Fred Brooks in The Mythical Man-Month, adding new personnel to an already-late project incurs a variety of risks and overhead costs that tend to make it even later; this is known as Brooks's law.
Another possible explanation is the "authorization imperative": Much of project planning takes place in a context where financial approval is needed to proceed with the project and the planner often has a stake in getting the project approved. This dynamic may lead to a tendency on the part of the planner to deliberately underestimate the project effort required. It is easier to get forgiveness (for overruns) than permission (to commence the project if a realistic effort estimate were provided.) Such deliberate underestimation has been named strategic misrepresentation.
Read more about this topic: Planning Fallacy
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