Five Things About Deterrence:
1. The certainty of being caught is a vastly more powerful deterrent than the punishment.
Research shows clearly that the chance of being caught is a vastly more effective deterrent than even draconian punishment.
2. Sending an individual convicted of a crime to prison isn’t a very effective way to deter crime.
Prisons are good for punishing criminals and keeping them off the street, but prison sentences (particularly long sentences) are unlikely to deter future crime. Prisons actually may have the opposite effect: Inmates learn more effective crime strategies from each other, and time spent in prison may desensitize many to the threat of future imprisonment.
3. Police deter crime by increasing the perception that criminals will be caught and punished.
The police deter crime when they do things that strengthen a criminal’s perception of the certainty of being caught. Strategies that use the police as “sentinels,” such as hot spots policing, are particularly effective. A criminal’s behavior is more likely to be influenced by seeing a police officer with handcuffs and a radio than by a new law increasing penalties.
4. Increasing the severity of punishment does little to deter crime.
Laws and policies designed to deter crime by focusing mainly on increasing the severity of punishment are ineffective partly because criminals know little about the sanctions for specific crimes.
More severe punishments do not “chasten” individuals convicted of crimes, and prisons may exacerbate recidivism.
See Understanding the Relationship Between Sentencing and Deterrence [1] for additional discussion on prison as an ineffective deterrent.
5. There is no proof that the death penalty deters criminals.
According to the National Academy of Sciences, "Research on the deterrent effect of capital punishment is uninformative about whether capital punishment increases, decreases, or has no effect on homicide rates."
[1] https://nij.gov/five-things/Pages/deterrence.aspx#addenda