Proceedings of the Annual Meeting of the Cognitive Science Society

Human estimates of the probabilities of combinations of events show well-established violations of probabilitytheory, most notably the conjunction and disjunction fallacies. These violations have led researchers to conclude that therules of probability are too complex for most people to use, and that cognitively-easier approximations such as averagingare used instead. Unlike previous work that has assumed that individuals use only a single combination rule, we collectedrepeated estimates of conjunctions and disjunctions and investigated whether individuals consistently used a single rule orused a repertoire of rules in a trial-by-trial Bayesian analysis. When not incentivized, most participants were best describedas randomly selecting a combination rule on each trial, and the correct rule was the most often used. Despite this, whenincentivized to use their single-best strategy participants were more likely to use the incorrect averaging rule. People do notseem to understand their own strategies well.