There are two additional situations that could further negate or at least diminish incapacitation. The first one relates to crime desistence, a phenomenon exhaustively documented in crime research. Simply put, as a result of aging, many offenders stop committing crimes, and thus past a certain point their incarceration yields no incapacitation benefits.
In the second case, incapacitation would initially be effective but would eventually become counterproductive, if as a result of an episode of incarceration, an offender upon release evolves into more serious crimes or engages in the same criminal behaviors but at substantially higher rates. In this instance prisons would have a criminogenic effect, preventing some crimes at first but at the expense of contributing to more serious or to a higher number of crimes in the future.
The incapacitative estimates presented before only indirectly attempt to control for replacement or desistence of offenders following incarceration, and none allow for the possibility that prisons are indeed criminogenic. These omissions further compound the uncertainties surrounding the available incapacitation estimates and show the need to develop yet better measuring techniques and substantially richer data sets.
I can't figure put who the hell he (or she! I have retained my lesson from the Digby out-coming for at least five days) is, but it sounds right to me. The association of crime with prison releases tend to confirm the school of crime hypothesis. Raphael and Stoll believe that the releasee is on average less criminally active than the average new prison commitment, but this doesn't tell you where the releasee would have been were it not for prison. Find a way to measure that one, egg-head.
Incidentally, Professor Sharkey of Boalt Law School, who lurks behind a Yale Law Jorunal pay-wall, confirms my previously expressed suspicion that the most prominent such research simply ignores crime in prison:
Studies of the effect of incarceration on crime rates usually ignore crime within prisons. See, e.g., Steve Levitt, The Effect of Prison Population Size On Crime Rates: Evidence from Prison Overcrowding Litigation (National Bureau of Economic Research Working Paper No. 5119, 1995); William Spelman, Criminal Incapacitation (1994); Franklin E. Zimring & Gordon Hawkins, The Scale of Imprisonment (1991) [hereinafter Imprisonment]; Thomas B. Marvell & Carlisle E. Moody, Jr., Prison Population Growth and Crime Reduction, 10 J. Quantitative Criminology 109 (1994). Crimes committed within the prison walls, if explicitly acknowledged in a utilitarian analysis of incapacitation, would reduce the calculated social benefit of crimes averted in society.