A case ofarttficialparanoia has been synthesized in the form of a computer simulation model. The model and its embodied theory are briefly described. Several excerpts from interviews with the model are presented to illustrate its paranoid input.output behavior. Evaluation of the success of the simulation will depend upon indistinguishability tests. Within the paradigm of computer science, distinctions are sometimes drawn between the activities of computer simulation and artificial intelligence. Yet in constructing models of psychological processes, the distinction can become blurred in places where overlaps emerge, as will be evident from our account of a model of artificial paranoia. I. Simulation ModeLq and Artifacts An information-processing system is defined as a structured combination of functions which collaborate in governing a set of input-output behaviors. I This research is supported by Grant PHS MH 06645-09 from the National Institute of Mental Health, by (in part) Research Scientist Award (No. I-KOS-K-!4, 433) from the National Institute of Mental Health to the senior author and by (in part) the Advanced Research Proj~ts Agency of the Office of the Secretary of Defense (SD-183). Artificial Intelligence 2 (1971), 1-25 Copyright © 19"/1 by North-Holland Publishing Company 2 K.M. COLBY, S. WEBER, F. D. HILF Two information-processing systems, Sx and S z , are considered input-output (I-O) equivalent when the I-O pairs of $1 in a particular situation are indistinguishable from the I-O pairs of Sz in a similar situation in respect to specified dimensions. To simulate the I-O behavior of a system, S1, one constructs a computer simulation model, Sz , whose I-O behavior imitates that of $1 a long tain dimensions. Our phrase 'artificial paranoia' refers to an actual but non-human case of paranoia which we have constructed in the form of a computer model. The model's I-O behavior, in the communicative situation of a diagnostic psychiatric interview, is identifiable by psychiatric judges as 'paranoid'. In constructing this paranoid model we were not attempting to simulate any actual human case of paranoia. Our artificial case is that of an imagined hypothetical individual. However, the model's I-O behavior imitates the I-O behavior of humans whose information processing is dominated by a mode psychiatrists label as 'paranoid'. This simulation model can be classified as a theoretical model in that it embodies as part of its inner structure an explanatory account of complex I-O paranoid behavior. It attempts to systematize and account for certain empirical regularities and particular occurrences familiar to clinicians who interview paranoid patients. An explanatory account involves functional relations expressed as lawlike generalizations. In order to explain concrete individual cases, it also contains initial conditions expressed as singular statements. Our model embodies general theoretical principles about paranoid communicative I-O behavior. In order to run and test the model as an explanation, these principles are combined with initial conditions descriptive of an individual hypothetical case. Our model of artificial paranoia represents a synthesized case of paranoid information processing. It is not an 'ideal' case either in the sense of an entity known to be impossible, such as a molecule without mass, or in the sense of an extreme type, such as absolute zero. Evaluation of the model as a successful simulation depends on a consensus of expert judgments by psychiatrists who interview it.