The utility of a warning to deter malingering on measures of personality and psychopathology was examined. Sixty-seven first year psychology students were randomly assigned to one of three conditions: unwarned malingerers, warned malingerers, and controls. Participants in the two malingering groups were given a financial incentive to simulate believable psychological impairment. Warned malingerers received an additional warning that the tests could detect malingering and that detection would result in loss of course credit. Controls received standardized test instructions. It was hypothesized that the malingering incentive would be sufficient to induce malingering, but that a deterrence theory warning would have a subsequent deterrent effect. Between-groups analyses indicated that the warning used in this study significantly altered test performance on the Personality Assessment Inventory (PAI) and revised Symptom Checklist 90 (SCL-90-R). Warned malingerers scored significantly lower (faked less) than unwarned malingerers on the majority of the psychopathology scales and frequently approximated control group performances. These results support the effectiveness of a warning to complement existing malingering detection methods. Copyright © 2009 John Wiley & Sons, Ltd.