[1] Criminal deterrence theory has two possible applications: the first is that punishments imposed on individual offenders will deter or prevent that particular offender from committing further crimes; the second is that public knowledge that certain offences will be punished has a generalised deterrent effect which prevents others from committing crimes.
When an offender is punished by, for example, being sent to prison, a clear message is sent to the rest of society that behaviour of this sort will result in an unpleasant response from the criminal justice system.
A key assumption underlying deterrence theory is that offenders weigh up the pros and cons of a certain course of action and make rational choices.
[5] Two utilitarian philosophers of the 18th century, Cesare Beccaria and Jeremy Bentham, formulated the deterrence theory as both an explanation of crime and a method for reducing it.
[9] Mental health and personality disorders will clearly have an impact of an individual's capacity to make rational decisions about their offending behaviour.
Adults with traumatic brain injury were first sent to prison when quite young and reported higher rates of repeat offending.
[10] Having a head injury also reduces an individual's capacity for rational decision making, and the same goes for Fetal alcohol spectrum disorder, a neurological disability of the brain.
Research has found that it causes "learning disabilities, impulsivity, hyperactivity, social ineptness, poor judgment, and can increase susceptibility to victimization and involvement in the criminal justice system".
[12] In order for a particular sanction to act as a deterrent, potential offenders must be aware of exactly what punishment they will receive before they commit an offence.
[13] Offenders are likely to be well aware that crimes such as assault, robbery, drug dealing, rape and murder will be punished but lack fine-grained knowledge of what the specific penalty is likely to be.
[citation needed] Another concern is that even if offenders have accurate knowledge about potential penalties, they do not necessarily take that information into account prior to committing a crime.
[24] Despite numerous studies using a variety of data sources, sanctions, crime types, statistical methods and theoretical approaches, there remains little agreement in the scientific literature about whether, how, under what circumstances, to what extent, for which crimes, at what cost, for which individuals and, perhaps most importantly, in which direction do various aspects of contemporary criminal sanctions affect subsequent criminal behavior.
[28] He argues that the issue is not whether the criminal justice system in itself prevents or deters crime but whether a new policy, added onto the existing structure, will have any additional deterrent effect.
[28] More recent research by Nagin (2009) found that increased severity of punishment had little deterrent effect on individual offenders.
[30] A meta-analysis of the deterrent effect of punishment on individual offenders also suggests little benefit is gained from tougher sentences.
In 2001 Canadian criminologist, Paul Gendreau, brought together the results of 50 different studies of the deterrent effect of imprisonment involving over 350,000 offenders.
[citation needed] The death penalty is still retained in some countries, such as in some parts of the United States, one reason being due to the perception that it is a deterrent to certain offenses.
In 1975, Ehrlich claimed the death penalty was effective as a general deterrent and that each execution led to seven or eight fewer homicides in society.
[32] A major difficulty in evaluating the effectiveness of the death penalty as a deterrent in the United States is that very few people are actually executed.
To read more about the argument concerning who deterrence is aimed at see Beccaria and Bentham's ideas as presented in Moberly, Sir W. H., [1968] The Ethics of Punishment.