Studies in subjective probability III: The unimportance of alternatives

In four experiments, student subjects were asked to estimate probabilities for a list of two to ten exhaustive, non-chance events, covering a variety of situations, both of prediction and diagnosis. Only in the two-alternative case a majority gave estimates which add up to unity (or 100%). As the number of alternatives increased, the total probability increased far beyond 100%, indicating a non-distributional probability concept. For instance, the probability that a person has committed murder was considered to be quite independent of his being one of three or one of five suspects. Even when subjects were told that the list should be extended with two additional alternatives and were allowed to correct their earlier estimates, few thought it necessary to do so, and corrections went equally in both directions.