Although 18 states and the District of Columbia have currently abolished the death penalty, more than 80 inmates were sentenced to death in 2013. A study recently published in the Proceedings of the National Academy of Sciences has revealed that one out of every 25 inmates on death row are most likely innocent in the crime they were convicted of.

“This is a disturbing finding,” lead researcher and law professor at the University of Michigan law school, Samuel Gross, told The Guardian. “There are a large number of people who are sentenced to death, and despite our best efforts some of them have undoubtedly been executed.”

Gross and his colleagues involved with the study used a scientific method that determines the effectiveness of certain medical treatments in reducing patient mortality rate known as “survival analysis.” Data included 7,482 inmates who were sentenced to death between January 1974 and December 2004. The information was used to estimate the percentage of defendants who would have been exonerated of their crimes and released from jail had they remained in prison.

Between 1973 and 2004, 138 U.S. prisoners on death row were exonerated of their crime and released from prison after they were proven innocent. Unfortunately, a large number of inmates given the death penalty were unable to prove their innocence in time. It is likely some of the 1,320 death row inmates who have been executed since 1977 were innocent, The Associated Press reported.

Proving an inmate is not guilty becomes problematic when they are removed from death row and given a life sentence. When this happens, their cases receive less attention compared to inmates who are still facing the death penalty. Over 60 percent of inmates on death row have been removed and given a life sentence. Researchers estimated that 4.1 percent of inmates who received the death penalty and remained in that status would have been exonerated of their crimes.

"The high rate of exoneration among death-sentenced defendants appears to be driven by the threat of execution," the study’s authors explained. "But most death-sentenced defendants are removed from death row and resentenced to life imprisonment, after which the likelihood of exoneration drops sharply."

Source: O’Brien B, Hu C, Kennedy E, Gross S. Rate of false conviction of criminal defendants who are sentenced to death. Proceedings of the National Academy of Sciences. 2014.