"The realization that mistakes have been made, that innocent people are still being freed, has made juries hesistant," Richard Dieter of the Death Penalty Information Center said. "They are willing to convict but not sentence to death. There is a demand for perfect proof, and so prosecutors are taking more plea bargains."
Seven death-row inmates were exonerated last year, the most since 2009.
A majority of Americans still support capital punishment. In a May poll by NBC News, 59 percent said they favor the death penalty as the ultimate punishment for murder, while 35 percent said they are opposed. That reflects the erosion of support since the 1990s, when more than 70 percent backed executions.
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