This work follows from the assumption that person perception processes allow people to categorize others, and, thereupon, to predict the perceived person's behaviors. A scale, the Mental Illness Behaviors Prediction Scale (MIBPS) was developed for use in studies of ascribed mental illness. The MIBPS is comprised of fifteen items, each of which describes a situation and four alternative behaviors scaled for “mental illness level.” The alternatives were clearly scaleable. High item-to-total-score correlations were found. When subjects rated a “very poorly adjusted person” and a “very well-adjusted person,” the item scores, as assigned to these two persons, were clearly differentiating. In other studies the overall “mental illness level” of perceived persons was found to vary with selected independent variables. The utility of the scale supports the conclusion that people have developed and do use a person-perceiving dimension labeled mentally ill/mentally healthy, and the use of this dimension promotes the expectation of specific kinds of behavior from the target person.