murders and 100,000 forcible-rape cases per year. It is estimated that 30% of all murder cases and 70% of all rape cases are never closed by arrest (John Hicks, personal communication, 1990). It should also be pointed out that only an estimated 50% of rapes are in fact even reported.
The second question is also difficult to answer, but it is clear that crimes of most types will not afford the opportunity to recover relevant biological evidence that will allow the police to identify an unknown suspect—i.e., the perpetrator's own body fluids. They include larcenies, burglaries, and assaults, for which ordinary fingerprints are frequently found. The major exception is rape, for which semen samples can be recovered in many cases and might provide prima facie evidence of sexual intercourse. In a small minority of homicides, blood, hair, or tissue samples from the perpetrator are left at the scene of the crime (e.g., because of a fight at the scene).
A DNA profile databank would thus be valuable primarily in investigating forcible rape, although the databank would be useful for some other investigations. State legislatures considering setting up such databanks should weigh the benefits in terms of solved rape cases and the costs in terms of collecting samples from persons likely to commit rapes (primarily, it seems, convicted sex offenders). Initial state efforts to develop DNA profile databanks were indeed aimed at sex offenders. Interestingly, some states rapidly expanded their programs to include all convicted offenders—without explicit weighing of the potential benefits of possessing such persons' patterns for solving crimes and the potential costs.
The above discussion justifies the development of a databank of DNA profiles of unknown subjects (open cases) and of offenders convicted of violent sex crimes. Such a databank would provide law enforcement with a powerful tool in linking sexual-assault cases through DNA profiles and tracking the activities of serial rapists. In light of recidivism and the continuing increase in reported rapes in this country, a databank of convicted sex offenders would provide investigators with a logical first place to look for assistance in solving unknown-offender sexual-assault cases.
DNA typing profiles of suspects might also be useful in associating a person with open or unsolved cases pending in other jurisdictions or states. Although a suspect's DNA profile might ultimately be entered into a convicted-felon databank, there would no doubt be a substantial period during which a suspect might engage in other criminal activities. Thus, in the case of a serial rapist, a person under suspicion and investigation for one offense, might be responsible for several later offenses for which he is not suspected. Therefore, if a DNA profile of a suspect is entered into a databank, it would be available to be searched against future unsolved cases.