The types of criminal offenses for which mentally retarded individuals are charged has seldom been the subject of empirical investigation. The prevalence of mentally retarded sexual offenders, in particular, remains unclear although the issue generates significant public concern as well as treatment challenges for professionals (1,2,3). Published estimates of retardation among sexual offenders vary considerably, with some authors reporting that retarded individuals are not over-represented among sex offenders (1). Others have suggested that sexually deviant behavior may be more common, or at least identified more often, among the mentally retarded population (2). Much of the research on mentally retarded sex offenders, however, has relied on data generated from subjects already incarcerated in prison (1,4,5). These data do not account for the substantial proportion of retarded individuals charged with a criminal offense, for example, who are considered incompetent to stand trial and are subsequently diverted away from the criminal justice system (6). Thus, data regarding the prevalence of retardation based upon incarcerated samples may substantially underestimate the true prevalence of sex offenses among retarded persons.
More accurate estimates of the proportion of sex offenders who are developmentally disabled may be obtained by analyzing pre-trial forensic evaluations. Evaluations of a subject's competence to stand trial and mental state at the time of the offense (criminal responsibility) are typically sought before plea bargaining or other diversionary procedures are initiated. Although impaired intellectual functioning, or the presence of a serious criminal offense, often prompt a forensic evaluation, the proportion of retarded defendants who are referred for evaluation is still likely to be an underestimate of the true prevalence of retardation among criminal defendants. Retarded defendants who cooperate with their attorneys or actively disguise their disabilities may not be identified as developmentally disabled (7,8). Nevertheless, a recent study of 103 pre-trial evaluations performed at a Michigan forensic facility revealed a disproportionately high rate of sex offenses among mentally retarded subjects, with sex offenses accounting for 35 percent of retarded defendants and only 10 percent of normal IQ defendants sampled (9).
Our study uses data gathered over a six year period through a statewide forensic evaluation system. We examine the relationship between mental retardation and criminal offense patterns, and in particular, the prevalence of mental retardation among individuals charged with a sex offense.
In 1981, the state of Virginia implemented a statewide forensic training program to enable community clinicians to perform outpatient .forensic evaluations (6). In 1985, the state began requiring clinicians to submit a forensic information form with their request for reimbursement. Since that time, these forms have been completed on most forensic evaluations performed in the state.
The 4,485 forms submitted between July 1, 1985, and June 30, 1991 included such data as the offense for which the defendant was charged, DSM-III (and DSM-III-R) diagnosis, and psycho-legal opinions of the clinician. Diagnoses were recorded in 60 percent of the cases (2,701 of 4,485) and an offense was recorded in 96 percent (4,302 of 4,485). The analyses below are based on the 2,536 cases in which both diagnostic and offense data were available.
For this study, offenses are grouped into six categories: homicide, crimes against people (i.e., robbery and assault), sex offenses, property crimes, public order offenses (i.e., trespassing and obstruction of justice) and all others (i.e., drug offenses and concealed weapons). In further analyses, crimes are grouped into six broader categories in order to compare retarded and non-retarded subjects charged with a sex offense. …