Study findings did not support the hypothesis that there would be neuropsychological impairment among sex offenders compared with nonsex offenders independent of the confounding variables examined in the study. This conclusion does not imply that deviant sexual behavior and preferences may not have roots in neurodevelopmental processes, but rather that the neuropsychological impairments may co-occur with the sexual disorder rather than being the direct cause of the deviant sexual behavior. Results confirmed the concerns of Leckliter and Matarazzo (1989) and Seltzer and Sherwin (1978) about the confounding variables that can influence cognitive impairment on such measures as the HR battery. Interpretation of results is made more difficult by individual differences in age, IQ/education, and lifestyle variables. When all of the significant variables were compared in stepwise regression analysis, age and IQ dominated results, but the presence of learning disorders and endocrine abnormalities remained significant variables in predicting HR impairment. When mental health professionals testify as expert witnesses on the role of potentially pre-existing factors in the neuropsychological makeup of the offender, this article recommends that all of the variables examined in this study--especially head trauma, endocrine disorders, substance abuse, and learning disorders--be assessed. This enables forensic clinicians to offer an informed opinion about the role of cognitive impairment in the defendant's offense. The samples were selected from a forensic database of men assessed with the HR battery between 1966 and 2006 at a university hospital and/or private clinic in a large urban Canadian community. 4 tables and 88 references