Prison crowding has pressed policymakers to a more efficient selection of offenders for incarceration. The theory of selective incapacitation argues that a small percentage of offenders commits a large percentage of crimes, so crime could be significantly reduced by identifying and imprisoning such offenders. The validity of this theory depends on the incapacitated offenders not being replaced by new offenders. The selection of habitual and serious offenders has been the focus of research by Jan and Marcia Chaiken and Peter Greenwood. Confirmation of the validity of this research for the selection of habitual offenders requires further studies. Also, the use of the selection instrument and the kinds of data required to administer it raise legal and philosophical questions. Some of these issues are the basing of sentences on predicted future crimes rather than the offense of conviction and the risk that the selection instrument may be flawed by design or information input. So long as selection is accurate and the assigned sentence does not exceed a reasonable maximum sentence for the offense of conviction, the use of a selection instrument to implement selective incapacitation can be legally and philosophically justified. An instrument, however, should never be applied mechanistically. Judicial discretion is required to deal with complex issues not encompassed by a selection instrument. 7 references.