Summary: The discriminant value of information that individualizes an object and indicates its category membership. This study examines with the tendency to neglect information pre-existing base rate in favor of individuating information. Subjects may have this tendency because they believe in the strong distinguishing value of individuating information. According to this hypothesis, the tendency to neglect information on pre-existing base rate should be weaken in two cases. First, when the subjects have no knowledge of the categories that are mentioned and are not able to estimate the distinguishing value of the individuating information. Second, when subjects are acquainted with the categories but when, according to their knowledge, the characteristics of the individuating information do not correspond to the ones they see as distinguishing. Hence, by retelling Tversky and Kahneman's story of the lawyers and engineers, we can manipulate the type of job categories with which the subjects are presented. The results of a first experiment enabled us to justify subjects' choices. A second experiment showed the impact of the job selection on the discrepancy between the subjects' estimates and the results of calculations of the base rate, supporting the hypothesis. Key words : individuating information, base rate, knowledge of the object, rationality.