No one can.
But, of the poorer states, the ones WITH capital punishment tend to have higher murder rates.
West Virginia, the 2ND poorest State in the US, has the 23rd highest Murder rate, and no Capital Punishment.
Maryland, the richest State in the US, has the 3rd highest murder rate, and HAS Capital Punishment.
So we have a poor State with a modest murder rate and no CP, and a very rich state with CP and a very high murder rate.
The correlation between capital punishment and murder rate suggests that NO Capital Punishment is a deterant, and Cp is not.
As for the punishment factor, I think 30 years of hard time is far worse than 10 followed by execution.
edit:
i understand the draw of CP, I used to be for it. But to argue for it using deterrence as an example of it's effectiveness is a bad idea. It just isn't there. I agree that CP satisfies the populace that the killer will never kill again, and there's no getting around that. But I think the driving force behind most proponents of CP is the vengeance factor, and like it or not, this is not an acceptable factor in our justice system. We can change this, sure, but as it stands, vengeance is not something the Candian Justice system considers. The US system does, but I question the motives of anyone who looks at their system, and especially their recidivism, and wishes to emulate their system.