Q8. Does the death penalty deter crime?

-YES

The number of murders declined by 20% in the State of Utah after the execution by firing squad of convicted killer, Gary Gilmore in 1977. 

A 2003 study by Emory University researchers of data from more than 3,000 countries from 1977 through 1996 found that each execution, on average, resulted in 18 fewer murders per country. 

Another examination, based on the data from all 50 states from 1978 to 1997, economist Paul Zimmerman demonstrated that each state execution deters an average of 14 murders annually.

-NO

Jeffrey Fagan, a professor of law at Columbia University in the U.S. said that there was no credible scientific evidence that the death penalty deters criminal behavior. Also added, "even when executions are frequent and well publicized, there are no observable changes in crime. Executions serve only to satisfy the urge for vengeance. Any retributive value is short lived, lasting only until the next crime." 

As you can see, there are yes and no to this question. Though the statistics are shown, there's no credible scientific evidence so neither can be said.

[CITATION]

・Utah State Police Journal.

・ABC. FactCheck. http://www.abc.net.au/news/2015-02-26/fact-check3a-does-the-death-penalty-deter3f/6116030

・The Daily Signal. http://dailysignal.com/2014/10/04/capital-punishment-works-it-deters-crime/