Lack of rationality Impact of alcohol and drugs In the
United States, one study found that at least half of all state prisoners are under the influence of alcohol or drugs at the time of their offence. The National Center for Drug Abuse Statistics estimates that twenty-six percent of US arrests are related to drug offenses.
Impact of mental health disorders Research shows that a significant proportion of those in prison have personality disorders or other mental health disorders which affect their ability to make rational decisions. A 2016 study in
Lancet Psychiatry has found that "prisoners have high rates of psychiatric disorders... Despite the high level of need, these disorders are frequently under-diagnosed and poorly treated". In 2002, a systematic review of 62 different studies from 12 different countries published in
The Lancet found 65% of men in prison and 42% of women have a personality disorder. Mental health and personality disorders will clearly have an impact of an individual's capacity to make rational decisions about their offending behaviour.
Impact of brain injury Many inmates have suffered head injuries, which can lead to loss of impulse control and cognitive impairment. A study in 2010 found that over 60% of prison inmates had experienced a significant head injury. Adults with traumatic brain injury were first sent to prison when quite young and reported higher rates of repeat offending. 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". In fact, youths with FASD are 19 times more likely to be incarcerated than those without FASD in a given year because of their poor decision-making.
Knowledge of likely punishment 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. However, evidence suggests that few people know what sentence will be imposed for a particular crime and, in the United States, most people generally underestimate how severe the sentence will be. 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. A study by Anderson (2002) found that only 22% of offenders convicted of cultivating cannabis "knew exactly what the penalties would be". That is not surprising given that sentencing is a complex process: what sanction is imposed depends on a number of different factors including the offender's age, previous criminal history, whether or not they plead guilty, their perceived level of remorse, and any other mitigating factors. If a potential offender does not know what punishment will be imposed, that undermines the ability to make a rational choice about whether the potential pain associated with committing a particular crime outweighs the potential gain. 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. Anderson found that 35% of offenders failed to think about the likely punishment prior to committing the offence.
Lack of certainty of punishment There are usually significant differences between the levels of crime in official statistics and the number of people who report they have been victimised in surveys of crime. In the United Kingdom, only an estimated 2% of offences lead to a conviction, and only one in seven of those convictions results in a prison sentence. The Home Office (1993) concluded that "the probability of being sent to prison for a crime is about one in 300". In the United States, it has been calculated that only one out of every 100 burglaries leads to a custodial sentence. In regard to drug use, the chances of getting caught are even more remote: less than one in 3,000. If it is unlikely that an offender will actually be caught, let alone punished, there is thus very little certainty of punishment, and any deterrent effect is substantially reduced.
Perceptions of risk Durrant (2014) argues that it is the
perception of risk that has the potential to deter offending rather than punishment itself. He cites a study of offenders in which 76% did not think about getting caught or thought the chances of getting caught were slim. Offenders who have successfully gotten away with certain crimes are especially likely to discount the probability of getting caught, particularly for drunk-driving. Durrant concludes: "for any given offence, the chances of actually getting punished by the criminal justice system are quite slim and active criminals are well aware of these favourable odds, thus undermining the potential deterrent effects of punishment".
Certainty vs. severity It is commonly assumed that increasing the severity of punishment increases the potential pain or cost of committing a crime and should therefore make offending less likely. One of the simplest methods to increase the severity is to impose a longer prison term for a particular crime. However, there are limits to how severe a punishment can be imposed because of the principle of
proportionality: the severity of the punishment should be roughly proportionate to the gravity of the offending. In a review of the literature, Durrant found that "most systematic reviews of the effects of sentencing severity on crime conclude, with a few exceptions, that there is little or no evidence that increasing the
punitiveness of criminal sanctions exerts an effect on offending". This is partly because many offenders get used to being in prison with the result that longer sentences are not necessarily perceived as being more severe than shorter sentences. Offenders who perceive that sanctions for particular crimes are almost inevitable are less likely to engage in criminal activity. However, because of low apprehension rates in most criminal justice systems, in practice it is much easier to make penalties more severe than it is to make them more certain. ==Effectiveness==