Deterrence

Deterrence is a utilitarian theory of punishment that aims to prevent crime by deterring potential criminals with heavy or highly visible consequences.

Effectiveness, or lack thereof, of punishments as deterrents
According to a Pew report, “increased use of incarceration accounted for one-quarter to one-third of the crime drop in the 1990s,” and, the report suggests, the country’s reached a “tipping point” where relying on more incarceration will no longer improve results. There is some evidence that prison prevents crime more through incapacitation than deterrence. Recidivism rates among those whose original crime was more serious were essentially unaffected by the length of their suspended sentence. An Australian study found that although imprisonment may have only a limited deterrent effect, the use of imprisonment can still be justified to achieve the purposes of punishment, denunciation, protection of the community or to provide time for rehabilitative treatment.

Criticisms
Murray N. Rothbard had several criticisms of deterrence based on its injustice and lack of proportionality.

On the lack of proportionality:


 * "If there were no punishment for crime at all, a great number of people would commit petty theft, such as stealing fruit from a fruit-stand. On the other hand, most people have a far greater built-in inner objection to themselves committing murder than they have to petty shoplifting, and would be far less apt to commit the grosser crime. Therefore, if the object of punishment is to deter from crime, then a far greater punishment would be required for preventing shoplifting than for preventing murder, a system that goes against most people's ethical standards. As a result, with deterrence as the criterion there would have to be stringent capital punishment for petty thievery--for the theft of bubble gum--while murderers might only incur the penalty of a few months in jail."

On its injustice:


 * "A classic critique of the deterrence principle is that, if deterrence were our sole criterion, it would be perfectly proper for the police or courts to execute publicly for a crime someone whom they know to be innocent, but whom they had convinced the public was guilty. The knowing execution of an innocent man--provided, of course, that the knowledge can be kept secret--would exert a deterrence effect just as fully as the execution of the guilty."

Private ownership of weapons as a crime deterrent
Regardless of whether any given citizen carries a gun, it is beneficial to him that there are others who carry them. From a standpoint of wanting to reduce the likelihood of crime below a certain level, it suffices that there is a sufficient possibility of any given person having a gun that enough criminals feel adequately deterred. Ironically, to the extent that the law successfully prevents minors from carrying guns, it would tend to make them that much more inviting targets for crime since, compared to certain groups (e.g. felons) they can be relatively easily identified as belonging to a forcibly disarmed class of citizens.