Characterizing Aggressive Behavior in a Forensic Population Tim R. Kockler, PhD, and Matthew S. Stanford, PhD Baylor University Chad E. Nelson, PsyD Florida State Hospital J. Reid Meloy, PhD University of California, San Diego Keith Sanford, PhD Baylor University The concept of a dichotomous versus a continuous aggression model continues to be debated within the research literature. The Impulsive/Premeditated Aggression Scale (IPAS; M. S. Stanford, R. J. Houston, C. W. Mathias, et al., 2003) is a newly developed self-report instrument designed to classify an individual’s aggressive behavior as predominantly premeditated or predominantly impulsive. The IPAS consists of 30-items that are scored on a 5-point Likert scale. This study used a nonrandom sample of convenience (N = 85) from a forensic state hospital. Principal-components analysis of the 30 items revealed 2 distinct factors (Impulsive and Premeditated Aggression), which accounted for 33% of the variance. The results of this study further validate the bimodal classification of aggression through its application to a forensic sample. The implications for general assessment, diagnosis, and treatment are discussed. Keywords: aggression, forensic, IPAS, impulsive, premeditated The ability to dichotomize aggressive behavior into distinct categories appears to be increasingly accepted in the empirical literature. Aggression has been defined in the broad-based litera- ture as premeditated (predatory, instrumental, callous– unemotional, proactive) and impulsive (affective, reactive, impul- sive; Cornell et al., 1996; Raine et al., 1998; Stanford, Houston, Mathias, et al., 2003; Weinshenker & Siegel, 2002; Woodworth & Porter, 2002). There is some empirical evidence supporting the validity and similarity of all of these typologies (Dodge & Coie, 1987), despite differences in chosen words to illustrate each type. According to clinical lore, premeditated–predatory aggression is carried out in a methodical and deliberate fashion for the benefit of achieving a desired goal. The heightened sense of awareness permits the perpetrator to home in on the victim, gathering all of the necessary information before carrying out the violent act. According to Cornell et al. (1996), an example of premeditated– predatory aggression is rape, especially serial rape, and offenders generally perform this violent act to gratify both psychopathic and narcissistic personality traits (Meloy, 2000). Studies show that during a premeditated–predatory mode of aggression, the aggres- sor typically shows very little, if any, physiological arousal (Stan- ford, Houston, Villemarette-Pittman & Greve, 2003), a hypothesis first proposed by Meloy (1988). These behavioral characteristics commonly associated with the premeditated–predatory aggressor make it very difficult for the victim to predict the impending attack (Meloy, 2000). Empirical studies on incarcerated populations have also demonstrated that premeditated–predatory aggressors are more psychopathic, as measured by the Psychopathy Checklist– Revised (PCL-R), than those classified as impulsive–affective aggressors (Cornell et al., 1996; Porter, Woodworth, Earle, Drugge, & Boer, 2003; Woodworth & Porter, 2002). Individuals who display impulsive–affective aggressive behav- iors are commonly labeled unpredictable and “short fused.” The impulsive–affective aggressor responds to provocation with imme- diate and destructive violence. The activation of the sympathetic branch of the autonomic nervous system provides the necessary means for accomplishing the ultimate goal of threat reduction (Meloy, 1988, 2000). It is quite plausible that the behavioral instability observed in the impulsive aggressor is related to cog- nitive dysfunction. For example, a few of the more recent scientific experiments found the impulsive aggressor to demonstrate signif- icant executive functioning and verbal impairments on neuropsy- chological testing (Stanford, Greve, & Gerstle, 1997; Villemarette- Pittman, Stanford, & Greve, 2002) and diminished P3 event- related potential amplitudes (Barratt, Stanford, Kent, & Felthous, 1997; Mathias & Stanford, 1999). Because of the lack of cognitive resources, the impulsive aggressor becomes overwhelmed by com- peting stimuli, which, if only for a brief moment, renders the aggressor helpless. Seeing no other alternative, the affective ag- gressor acts before he or she thinks, drawing on primal aggressive knowledge; consequently, affective aggressors are frequently caught and sent to jail for their violent outbursts (Meloy, 2000). Within the past 30 years, there have been attempts to validate psychological measures that would adequately tap the aggression construct (Barratt, Stanford, Dowdy, Liebman, & Kent, 1999; Tim R. Kockler, Matthew S. Stanford, PhD, and Keith Sanford, Depart- ment of Psychology and Neuroscience, Baylor University; Chad E. Nelson, Civil Forensic Services, Florida State Hospital, Chattahoochee, Florida; Reid Meloy, Department of Psychiatry, University of California, San Diego. For reprints and correspondence: Tim R. Kockler, PhD, Dixie Regional Medical Center, 544 South 400 East, St. George, UT 84770. E-mail: tk4nsic@yahoo.com American Journal of Orthopsychiatry Copyright 2006 by the American Psychological Association 2006, Vol. 76, No. 1, 80 – 85 0002-9432/06/$12.00 DOI: 10.1037/0002-9432.76.1.80 80