The weighing of pathological and non-pathological information in clinical judgment Yoav Ganzach * Faculty of Management, Tel Aviv University, 69102 Tel Aviv, Israel Received 17 May 1999; received in revised form 18 October 1999; accepted 19 October 1999 Abstract On the basis of the classic data of Meehl (1959), I examine how clinical psychologists use the MMPI scales to judge the degree of pathology of psychiatric patients by comparing linear models of the judgment to a linear model of the criterion (the actual diagnosis of the patients). This comparison reveals that excessively heavy weight is assigned to pathological information in comparison to non-pathological information. Additional analyses reveal that this biased weighing also in¯uences the actual diagnosis and that it is a major determinant of the accuracy of clinical judgment. It is suggested that these eects arise from a con®rmation bias associated with the hypothesis that a patient has severe, rather than mild, pathology. Ó 2000 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2340; 3040; 3310 Keywords: Clinical-judgment; Decision-making; Psycho-diagnonsis 1. Introduction During their career people go through a professional socialization process in which they learn the theories of their ®eld and adopt its metaphysical assumptions. This process de®nes for the members of the profession the phenomena that are Acta Psychologica 104 (2000) 87±101 www.elsevier.com/locate/actpsy * Tel.: +11-972-3640-9959; fax: +11-972-3640-9560. E-mail address: yoavgn@post.tau.ac.il (Y. Ganzach). 0001-6918/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 1 - 6 9 1 8 ( 9 9 ) 0 0 0 5 5 - 4