The theory of deterrence states that, "if the consequence of committing a crime outweighs the benefit of the crime itself, the individual will be deterred from committing the crime (Associated Content). http://www.associatedcontent.com/article/32600/evolution_of_deterrence_crime_theory_pg2.html?cat=37>.
detterence
The stated purpose of the death penalty is supposed to be deterrence; people will be less likely to commit such crimes for fear of being executed. This doesn't work in the USA, because the death penalty is so seldom carried out these days that criminals don't actually expect to be executed. The effectiveness of deterrence is directly proportional to the probability of the punishment being imposed. If murderers were actually executed instead of living for 20 years on "death row", it would be a more effective deterrent.