A heuristic whereby people make predictions, assess the probabilities of events, carry out counterfactual reasoning, or make judgements of causality through an operation resembling the running of a simulation model. The ease with which the mental model reaches a particular state may help a decision maker to judge the propensity of the actual situation to reach that outcome. The Israeli psychologists Daniel Kahneman (born 1934) and Amos Tversky (1937–96), who introduced the heuristic in a lecture in 1979 and published it as a book chapter in 1982, provided empirical evidence that people use it to predict the behaviour of others in certain circumstances and to answer questions involving counterfactual propositions by mentally undoing events that have occurred and then running mental simulations of the events with the corresponding input values of the model altered. For example, when provided with a vignette describing two men who were delayed by half an hour in a traffic jam on the way to the airport so that both missed flights on which they were booked, one of them by half an hour and the second by only five minutes (because his flight had been delayed for 25 minutes), 96 per cent of a sample of students thought that the second man would be more upset. Kahneman and Tversky argued that this difference could not be attributed to disappointment, because both had expected to miss their flights, and that the true explanation was that the vignette invited the use of the simulation heuristic, in which it would be easier to imagine minor alterations that would have enabled the second man to arrive in time for his flight. The heuristic is often interpreted as a form of availability heuristic.