WASHINGTON (AP) - When jurors choose a death sentence in cases that are not among the "worst of the worst," the sentence is more likely to be overturned on appeal, a study spanning 23 years of court records found.
Overall, states and counties where juries or judges impose the death penalty most often also tend to have the highest number of cases overturned because of errors or problems at trial, says the study being released Monday.
All but one of the 10 states with the highest death-sentencing rates had those sentences reversed as often or more often than the average rate nationally, said James Liebman, a Columbia University law professor and the study's lead author.
Death sentences are most often overturned because lawyers performed poorly at trial, prosecutors kept legitimate evidence out of the trial or judges gave flawed instructions to the jury, Liebman said his research showed.
The report found a state or federal court threw out a conviction or death sentence in 68 percent of the cases it studied in which at least one round of appeals had been completed. The study looked at 5,760 cases in the 34 states where the death penalty is actively applied.
Other researchers attacked findings in an earlier study on reversals by the same authors, and one review concluded that death sentences were really only overturned in 52 percent of cases or less.
The Criminal Justice Legal Foundation took issue with Liebman's central premise that errors are responsible for overturned sentences.
It's not new or surprising that a large percentage of capital verdicts are overturned, the foundation said. It said the issue is whether that happens because of mistakes, as death-penalty opponents contend, or because of unreasonable obstructions placed in the way of such sentences, as advocates of capital punishment assert.
Liebman's study does not take a position on whether the death penalty is ever appropriate. But in an interview, he said his research shows that if they are imposed at all, death sentences should be reserved for the worst cases.
"Imposing the death penalty in cases that are not the worst of the worst is a recipe for unreliability and error," the report said.
The greater the number of what courts call aggravating factors in a given capital crime, the less likely it is that a death sentence will be overturned, Liebman said. Aggravating factors can include whether the victim was a police officer, for example, or whether the killing was especially gruesome.
The reverse is also true, Liebman said. The more factors that tend to lessen the severity of a crime, the more likely it is that any death sentence imposed will be overturned.
At the final stage of court review, Liebman found that for each additional aggravating factor, the likelihood of reversal dropped 15 percent.
All states that allow the death penalty have laws making an appeal automatic. South Carolina, however, allows defendants to waive the right to appeal.
There have been 759 executions since they resumed in 1977 after a Supreme Court moratorium. Courts have ordered the release of 99 wrongly convicted death row inmates.