The state has a responsibility to protect the lives of innocent citizens, and enacting the death penalty may save lives by reducing the rate of violent crime. The reasoning here is simple- fear of execution can play a powerful motivating role in convincing potential murderers not to carry out their acts. While the prospect of life in prison may be frightening, surely death is a more daunting prospect. Thus, the risk of execution can change the cost-benefit calculus in the mind of murderers-to be so that the act is no longer worthwhile for them1. Numerous studies support the deterrent effect of the death penalty. A 1985 study by Stephen K. Layson at the University of North Carolina showed that a single execution deters 18 murders. Another influential study, which looked at over 3,054 counties over two decades, further found support for the claim that murder rates tend to fall as executions rise2. On top of this, there are ways to make the death penalty an even more effective deterrent than it is today. For instance, reducing the wait time on death row prior to execution can dramatically increase its deterrent effect in the United States1. In short, the death penalty can- and does- save the lives of innocent people. 1 Muhlhausen, David. "The Death Penalty Deters Crime and Saves Lives," August 28,2007. Accessed June 5, 2011.  2 Liptak, Adam. "Does Death Penalty Save Lives? A New Debate." The New York Times. November 18, 2007. Accessed June 9, 2011 
There are many reasons to doubt the deterrent effect of the death penalty. For one thing, many criminals may actually find the prospect of the death penalty less daunting (and thus, less effective as a deterrent) than spending the rest of their lives suffering in jail. Death by execution is generally fairly quick, while a lifetime in prison can be seen as a much more intensive punishment. Moreover, even if criminals preferred life in prison to the death penalty, it's not clear that a harsher punishment would effectively deter murders. Heinous crimes often occur in the heat of the moment, with little consideration for their legal repercussions1. Further, for a deterrent to be effective, it would have to be immediate and certain. This is not the case with the death penalty cases, which often involve prolonged appeals and sometimes end in acquittals2. Finally, the empirical evidence regarding the deterrence effect of the death penalty is at best mixed. Many of the studies that purport to show the deterrence effect are flawed, because the impact of capital punishment cannot be disentangled from other factors such as broader social trends, economic factors and demographic changes in a region2. Other studies have even suggested a correlation between the death penalty and higher crime rates. States such as Texas and Oklahoma, which have very high execution rates, also have higher crime rates than most states that do not have the death penalty2. 1 Amnesty International. "Abolish the Death Penalty." Accessed June 5, 2011.  2 "Saving Lives and Money." The Economist. March 12, 2009. Accessed June 5, 2011.