The utility of the MMPI-2 malingering discriminant function index in the detection of malingering: a study of criminal defendants

Detecting malingering mental illness in forensics: Known-Group Comparison and Simulation Design with MMPI-2, SIMS and NIM

BACKGROUND

Criminal defendants may often exaggerate psychiatric symptoms either to appear non-accountable for their actions or to mitigate their imprisonment. Several psychometric tests have been proposed to detect malingering. These instruments are often validated by Simulation Design (SD) protocols, where normal participants are explicitly requested to either simulate a mental disorder or respond honestly. However, the real scenarios (clinical or forensic) are often very challenging because of the presence of genuine patients, so that tests accuracy frequently differs from that one obtained in well-controlled experimental settings. Here we assessed the effectiveness in criminal defendants of three well-known malingering-detecting tests (MMPI-2, SIMS and NIM) by using both Known-Group Comparison (KGC) and Simulation Design (SD) protocols.

METHODS

The study involved 151 male inmates. Participants to the KGC protocol were all characterized by a positive psychiatric history. They were considered as genuine patients (KGC_Controls) if they had some psychiatric disorders already before imprisonment and scored above the cutoff of SCL-90-R, a commonly used test for mental illness, and as suspected malingerers (KGC_SM) if they were diagnosed as psychiatric patients only after imprisonment and scored below the SCL-90-R cutoff. Participants to SD protocol had no history of psychiatric disease and scored below the SCL-90-R cutoff. They were randomly assigned to either group: Controls (requested to answer honestly, SD_Controls) and simulated malingerers (requested to feign a psychiatric disease, SD_SM). All participants were then submitted to MMPI-2, NIM and SIMS.

RESULTS

Results showed that while MMPI-2, SIMS and NIM were all effective in discriminating malingerers in the SD, SIMS only significantly discriminated between KGC_Controls and KGC_SM in the Known-Group Comparison. Receiver Operating Characteristic (ROC) curves analysis confirmed the better sensitivity of SIMS with respect to the other tests but raised some issues on SIMS specificity.

DISCUSSION

Results support the sensitivity of SIMS for the detection of malingering in forensic populations. However, some specificity issues emerged suggesting that further research and a good forensic practice should keep into account multiple measures of malingering, including psychometric data, clinical and social history and current clinical situation. These methodological constraints must be kept in mind during detection of malingering in criminal defendants reporting psychiatric symptoms.

A Comparative Analysis of MMPI-2 Malingering Detection Models Among Inmates

There are several strategies, or models, for combining the Minnesota Multiphasic Personality Inventory-2 (MMPI-2) validity indicators to detect malingered psychiatric symptoms. Some scholars have recommended that an elevated F (Infrequency) score should be followed by the inspection of Fp (Infrequency—Psychopathology), whereas a recent meta-analysis indicated that Fp and Ds (Gough’s Dissimulation Scale) should be examined. For correctional settings, one model of malingering suggests that F, Fp, and F - K (Gough’s Dissimulation Index) should be inspected for one or more elevated scores. Although a large body of research has examined malingering detection with the MMPI-2, this research has typically focused on the contributions of individual validity indicators to differentiate malingered from genuine psychiatric symptoms. Therefore, the current study compared these models of malingering detection on the MMPI-2. Inmate simulators were contrasted with inmates who were hospitalized for psychiatric treatment. Results from classification and logistic regression analyses supported the sequential use of F and Fp in malingering detection.