"Science does really draw a conclusion. It did. There is no question about it," said Naci Mocan, an economics professor at the University of Colorado at Denver. "The conclusion is there is a deterrent effect."
A 2003 study he co-authored, and a 2006 study that re-examined the data, found that each execution results in five fewer homicides, and commuting a death sentence means five more homicides. "The results are robust, they don't really go away," he said. "I oppose the death penalty. But my results show that the death penalty (deters) — what am I going to do, hide them?"
Statistical studies like his are among a dozen papers since 2001 that capital punishment has deterrent effects. They all explore the same basic theory — if the cost of something (be it the purchase of an apple or the act of killing someone) becomes too high, people will change their behavior (forego apples or shy from murder).
I agree with these conclusions, although my main deterrence argument is more general. Having a death penalty sends a general message to society that crime will not be tolerated, and thus reduces criminal behavior more broadly than just murder.