The elusive wishful thinking effect

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Abstract
We define a desirability effect as the inflation of the judged probability of desirable events or the diminution of the judged probability of undersirable events. A series of studies of this effect are reported. In the first four experiments, subjects were presented with visual stimuli (a grid matrix in two colours, or a jar containing beads in two colours), and asked to estimate the probability of drawing at random one of the colours), and asked to estimate the probability of drawing at random one of the colours. The estimated probabilities for a defined draw were not higher when the draw entailed a gain than when it entailed a loss. In the fifth and sixth experiments, subjects read short stories each describing two contestants competing for some desirable outcome (e.g. parents fighting for child custody, or firms bidding for a contract). Some judged the probability that A would win, others judged the Desirability that A would win. Story elements that enhanced a contestant's desirability did not cause the favoured contestant to be judged more likely to win. Only when a contestant's desirability was enhanced by promising the subject of payoff contingent on that contestant's victory was there some slight evidence for a desirability effect: contestants were judged more likely to win when the subject expected a monetary prize if they won than when the subject expected a prize if the other contestant won. In the last experiment, subjects estimated the probability of an over-20-point weekly change in the Dow Jones average, and were promised prizes contingent on such a change either occurring, or failing to occur. They were also given a monetary incentive for accuracy. Subjects who desired a small change. We conclude that desirability effects, when they exist, operate by biasing the evidence brought to mind regarding the event in question, but when a given body of evidence is considered, its judged probability is not influenced by desirability considerations.

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