* Kahneman and Tversky originally explained the fallacy by envisaging that planners focus on the most optimistic scenario for the task, rather than using their full experience of how much time similar tasks require.  [4]
* Roger Buehler and colleagues account for the fallacy by examining  wishful thinking  ; in other words, people think tasks will be finished quickly and easily because that is what they want to be the case.  [6]
* 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.  [6]  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,  [12]  which is similar to the concepts outlined in  impression management  theory.
* Another explanation proposed by Roy and colleagues is that people do not correctly recall the amount of time that similar tasks in the past had taken to complete; instead people systematically underestimate the duration of those past events. Thus, a prediction about future event duration is biased because memory of past event duration is also biased. Roy and colleagues note that this memory bias does not rule out other mechanisms of the planning fallacy.  [13]
* Sanna and colleagues examined temporal framing and thinking about success as a contributor to the planning fallacy. They found that when people were induced to think about a deadline as distant (i.e., much time remaining) vs. rapidly approaching (i.e., little time remaining), they made more optimistic predictions and had more thoughts of success. In their final study, they found that the ease of generating thoughts also caused more optimistic predictions.  [10]
* One explanation,  focalism  , proposes that people fall victim to the planning fallacy because they only focus on the future task and do not consider similar tasks of the past that took longer to complete than expected.  [14]
* 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  .
* The "authorization imperative" offers another possible explanation: much of project planning takes place in a context which requires financial approval 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 by Jones and Euske "strategic misrepresentation".  [15]
* Apart from psychological explanations, the phenomenon of the planning fallacy has also been explained by  Taleb  as resulting from natural  asymmetry  and from  scaling  "Scale \(ratio\)") issues. The asymmetry results from random events giving negative results of delay or cost, not evenly balanced between positive and negative results. The scaling difficulties relate to the observation that consequences of disruptions are not  linear  , that as size of effort increases the error increases much more as a natural effect of inefficiencies of larger efforts' ability to react, particularly efforts that are not divisible in increments. Additionally this is contrasted with earlier efforts being more commonly on-time (e.g. the  Empire State Building  ,  The Crystal Palace  , the  Golden Gate Bridge  ) to conclude it indicates inherent flaws in more modern planning systems and modern efforts having hidden fragility. (For example, that modern efforts – being computerized and less localized invisibly – have less insight and control, and more dependencies on transportation.)  [16]
* Bent Flyvbjerg and Dan Gardner write that planning on government-funded projects is often rushed so that construction can begin as soon as possible to avoid later administrations undoing or cancelling the project. They say a longer planning period tends to result in faster and cheaper construction.  [17]