Patrice Rusconi, Marco Marelli, Selena Russo, Marco D’Addario, Paolo Cherubini

In two studies, we investigated how people use base rates and the presence versus the absence of new information to judge which of two hypotheses is more likely. Participants were given problems based on two decks of cards printed with 0-4 letters. A table showed the relative frequencies of the letters on the cards within each deck. Participants were told the letters that were printed on or absent from a card the experimenter had drawn. Base rates were conveyed by telling participants that the experimenter had chosen the deck by drawing from an urn containing, in different proportions, tickets marked either 'deck 1' or 'deck 2'. The task was to judge from which of the two decks the card was most likely drawn. Prior probabilities and the evidential strength of the subset of present clues (computed as 'weight of evidence') were the only significant predictors of participants' dichotomous (both studies) and continuous (Study 2) judgments. The evidential strength of all clues was not a significant predictor of participants' judgments in either study, and no significant interactions emerged. We discuss the results as evidence for additive integration of base rates and the new present information in hypothesis testing.