It has long been recognized that, while many offenders have only a limited or occasional involvement in crime, a small group of persistent offenders is responsible for a disproportionately large volume of crime. The question of whether persistent offenders repeatedly engage in a particular type of crime (offence specialization) or commit a diverse range of offences (offence versatility) has in turn generated considerable research attention. Persistent offenders have generally been found to be versatile in their criminal activity, although offence specialization has also been observed to varying degrees for certain offence types. Persistence and offence specialization are implied in much of the existing theoretical and empirical literature on sexual offending, but researchers have only recently begun to systematically test these assumptions.

The present thesis presents three empirical studies that together aim to contribute to emerging knowledge concerning offence specialization and versatility among sexual offenders. All three studies are based on a sample of 566 sexual offenders who were referred to the Massachusetts Treatment Center for Sexually Dangerous Persons (MTC) in Bridgewater, Massachusetts between 1959 and 1984. Approximately half (n = 315) (the ‘observed’ group) were assessed as not sexually dangerous, and released either into the community or back into prison to serve the remainder of their sentence. The remainder (n = 251) (the ‘committed’ group) were adjudged to be sexually dangerous, civilly committed, and eventually released. Criminal histories, MTC assessment data, and post-release recidivism data were obtained...