Is Capital Punishment Effective?

Alex Massie finds little reason to believe it is:

There does not seem to be much evidence showing that states that use capital punishment have lower murder rates than those that don't. On the contrary, in America states that still execute prisoners tend to have higher murder rates than those that do not. (Evidently this does not suggest that abolishing capital punishment lowers murder either. There seems little causation at play here.) Texas has executed nearly five hundred people since 1976 but its murder rate remains much the same as California's where, despite a large population on death row, few executions have actually been carried out in recent years.)