First, research shows clearly: If criminals think there’s only a slim chance they will be caught, the severity of punishment — even draconian punishment — is an ineffective deterrent to crime. Second, prisons are good for punishing criminals and keeping them off the street, but prison sentences are unlikely to deter future crime. Third, the police deter crime when they do things that strengthen a criminal’s perception of the certainty of being caught. Fourth, laws and policies designed to deter crime are ineffective partly because criminals know little about the sanctions for specific crimes. Fifth, 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."
Five Things About Deterrence
National Institute of Justice