argued that persons with schizophrenia reason in an orga- nized fashion but by different rules of logic. In particular, he claimed that such persons show a flawed use of predicates, failing to distinguish between essential and nonessential properties. In practical terms this difference entails taking any two objects with a shared predicate (both green, for in- stance, or both scary, or both starting with the letter p)tobe identical. The textbook case is of a patient who believes that sheistheVirginMarybecausebothsheandtheVirginMary are virgins. More colorfully, Von Domarus provided the ex- ample of a patient who associated Jesus with a cigar box. The connection was seemingly inexplicable until he realized that both Jesus and the cigar box had the property of being surrounded: Jesus by a halo, the cigar box by a state tax band. Von Domarus’s work evolved out of an interest already present in the field regarding the nature of inferences of identity among patients with schizophrenia. Goldstein 3,4 emphasized that the deficit was in identifying the essential qualities common to different objects. Hyman 5 later did re- search on patients’ identity judgments based on partial con- REVIEW Logical Processing, Affect, and Delusional Thought in Schizophrenia Lilianne Rivka Mujica-Parodi, PhD, Dolores Malaspina, MD, and Harold A. Sackeim, PhD Deficits of logical reasoning have long been considered a hallmark of schizophrenia and delusional disorders. We provide a more precise characterization of “logic” and, by extension, of “deficits in logical reasoning.” A model is offered to categorize different forms of logical deficits. This model acknowledges not only problems with making infer- ences, which is how logic deficits are usually conceived, but also problems in the acquisi- tion and evaluation of premises (i.e., filtering of “input”). Early (1940–1969) and modern (1970–present) literature on logical reasoning and schizophrenia is evaluated within the context of the presented model. We argue that, despite a substantial history of interest in the topic, research to date has been inconclusive on the fundamental question of whether patients with delusional ideation show abnormalities in logical reasoning. This may be due to heterogeneous definitions of “logic,” variability in the composition of patient samples, and floor effects among the healthy controls. In spite of these difficulties, the available evidence suggests that deficits in logical reasoning are more likely to occur due to faulty assessment of premises than to a defect in the structure of inferences. Such deficits seem to be provoked (in healthy individuals) or exacerbated (in patients with schizophrenia) by emotional content. The hypothesis is offered that delusional ideation is primarily affect-driven, and that a mechanism present in healthy individuals when they are emotionally challenged may be inappropriately activated in patients who are delusional. (HARVARD REV PSYCHIATRY 2000;8:73–83.) Although Kraepelin 1 identified cognitive abnormalities as one of the characteristics of schizophrenia when he first de- scribed the disorder, research specifically relating logic defi- cits to schizophrenia only gained prominence with the work of Von Domarus nearly half a century later. Von Domarus 2 From the departments of Medical Genetics (Drs. Mujica-Parodi and Malaspina) and Biological Psychiatry (Dr. Sackeim), New York State Psychiatric Institute, and the Department of Psychiatry, College of Physicians and Surgeons, Columbia University (Drs. Mujica-Parodi, Malaspina, and Sackeim), New York, N.Y. Original manuscript received 9 August 1999, accepted for publica- tion 28 September 1999; revised manuscript received 24 January 2000. Reprint requests: L. R. Mujica-Parodi, PhD, Department of Medical Genetics, Unit 6, New York State Psychiatric Institute, 1051 River- side Drive, New York, NY 10032.  2000 President and Fellows of Harvard College 73