The Death Penalty Information Center's 2008 Year End Report also notes the number of executions dropped this year to a 14-year low of 37, a statement said.
The group also aid Ohio was the only state outside the South to carry out an execution this year.
The Washington group said it estimates that the number of people sentenced to death in 2008 will be 111, the lowest number since the death penalty was reinstated in 1976.
The report said more than 25 executions were stayed in 2008, while four inmates were exonerated and four others received sentence commutations.
"Courts, legislatures and the public are increasingly skeptical about the death penalty, whether those concerns are based on innocence, inadequate legal representation, costs or a general feeling that the system isn't fair or accurate," said DPIC Executive Director Richard Dieter.