To our knowledge, this study establishes for the first time the existence of a subtle structural deficit in the prefrontal cortex of uninstitutionalized antisocial, violent persons with psychopathic-like behavior who live in community settings, and represents the first MRI findings on APD. It also extends previous neurological research that has observed pseudopsychopathic behavior in patients with neurological disorders with observable lesions affecting both gray and white matter by showing that a much less observable volume reduction specific to prefrontal gray matter is associated with APD in this community sample. The APD group had an 11.0% reduction in prefrontal gray matter compared with the control group, a 13.9% reduction compared with the substance-dependent group, and a 14.0% reduction compared with the psychiatric control group, with effect sizes corresponding to d = 0.76, 0.78, and 0.84, respectively, for absolute gray matter volumes and d = 0.83, 0.76, and 1.1, respectively, for prefrontal gray matter/whole brain volumes. Nevertheless, while these effect sizes are thought to be large,28 this deficit is visually imperceptible at a clinical radiological level, with group differences translating to less than half a pixel (0.5 mm) in the thickness of gray matter in any coronal prefrontal slice. Reduced autonomic activity during a social stressor was also observed, with large effect sizes in comparison to all 3 control, substance-dependent, and psychiatric control groups for skin conductance of 0.81, 0.80, and 0.79 microsiemens, respectively, and for a heart rate of 1.07, 0.87, and 0.91 beats per minute, respectively. Furthermore, persons with APD who had prefrontal gray matter volume reductions had lower skin conductance activity during the stressor than those without reduced prefrontal gray volume (d = 1.04).