Liberals like O'Malley consider ignorance to be a badge of honor. The death penalty is definitely a deterrent to capital crimes, other crimes not much. But then, no sensible person would expect the death penalty to deter crimes that are not punishable by death in the first place.
In 1972 the SCOTUS ruled out capital punishment nationwide in Furman v Georgia. After Georgia (and other states) corrected discrimination problems, the death penalty was reinstated in Gregg v Georgia 1976.
During the four years from 1972 to 1976, capital crimes increased dramatically. In the years following the 1976 decision, they dropped back down to their previous baseline. The two cases had set up a compelling, naturally controlled experiment.
The data may have impacted the Court because in subsequent cases, regarding execution of mentally handicapped and minors, the Court cited deterrence effects as a significant factor in their thinking.