The essential features are various oddities of thinking, perception, communication, and behavior not severe enough to meet the criteria for schizophrenia. No single feature is invariably present. The disturbance in thinking may be expressed as magical thinking, ideas of reference, or paranoid ideation. Perceptual disturbances may include recurrent illusions, depersonalization, or derealization. Often there are marked peculiarities in communication; concepts may be ex- pressed unclearly or oddly, using words deviantly, but never to the point of loosening of associations or incoherence. Frequently, but not invariably, the behavioral manifestations include social isolation and constricted or inappropriate affect that interferes with rapport in face-to-face interaction.