The central purposes of the death penalty, according to its proponents, are retribution, deterrence, and incapacitation.
None justifies the execution of persons with mental retardation.
Imposing the death penalty on persons with mental retardation is also not necessary to advance the putative goal of deterrence. If offenders with mental retardation were excluded from capital punishment, other offenders would still be liable to the death penalty. Finally, the death penalty is not the only means available to society to incapacitate people who pose a danger to society. Secure confinement of dangerous mentally retarded offenders adequately protects public safety. For example, as a recent New York Times analysis of state homicide rates revealed during the last 20 years, the homicide rate in states with the death penalty has been 48 percent to 101 percent higher than in states without the death penalty.