National Academy of Sciences Study: Over Four Percent of People Sentenced to Death are Likely Innocent

New peer-reviewed research indicates that at least 4.1 percent of defendants sentenced to death in the United States are likely innocent. The article, “Rate of False Conviction of Criminal Defendants who are Sentenced to Death,” published today in one of the world’s most respected scientific journals—Proceedings of the National Academy of Sciences—is available at http://www.eurekalert.org/account.php

“This study provides the first rigorous estimate of the rate of conviction of innocent criminal defendants in any context. It shows that the number of innocent people sentenced to death is more than twice the number of inmates actually exonerated and freed by legal action,” said Bruce Levin, Ph.D., Professor and Past Chair, Department of Biostatistics, Mailman School of Public Health Columbia University. Levin, an expert in statistics, did not participate in the research but is familiar with the study.

The rate of erroneous conviction of innocent defendants has been considered “unknowable.” However, the authors of the study used survival analysis, commonly used in medicine, to determine the 4.1% finding, which they consider a “conservative estimate.”

While 1.6 percent of those sentenced to death since 1973 in the U.S.—138 prisoners—have been exonerated and released due to innocence, many other innocent capital defendants are missed and not exonerated. Professor Samuel R. Gross of the University of Michigan Law School, the lead author of the study, notes, “The great majority of innocent people who are sentenced to death are never identified and freed. The purpose of our study is to account for the innocent defendants who are not exonerated.”

Numerous factors contribute to the failure to exonerate innocent capital defendants. Most sentences of death row prisoners are reduced after appeals to life in prison. While this saves the prisoners’ lives, the reduction in sentencing makes these cases much lower in priority for the time and attention lawyers, courts, and governors focus on capital cases that carry the pending risk of executing innocent people.

Consequently, while an innocent defendant might be exonerated if he were to remain on death row, he both avoids execution and is likely to die in prison once the sentence is reduced to life.

The focus of resources on capital cases is reflected in the fact, included in the report, that death sentences “represent less than one-tenth of 1% of prison sentences in the United States, but they accounted for about 12% of known exonerations of innocent defendants from 1989 through early 2012.”

This disproportion of more than 130 to 1 is also a result of other factors unique to capital cases. Almost all are based on convictions after a jury trial, unlike the vast majority, approximately 95%, of felony convictions that are negotiated in plea bargains. Most defendants in the latter category are not represented by an attorney post-conviction and most of the cases are not reviewed. All death penalty cases are subject to sentencing hearings, a significant appeal process, and much greater scrutiny than other cases. Thus the opportunity to discover conviction error is much greater in capital cases.

The report notes, “Courts and executive officials explicitly recognize that it is appropriate to take the possibility of innocence into account in deciding whether to reverse a conviction for procedural error or commute a death sentence to life imprisonment, and a wealth of anecdotal evidence suggests that this practice is widespread. As a result, those who are resentenced to punishments less than death are more likely to be innocent than those who remain on death row.”

While far greater attention and opportunity for exoneration occurs in death-row cases, the study concludes, “[N]o process of removing potentially innocent defendants from the execution queue can be foolproof. With an error rate at trial over 4%, it is all but certain that several of the 1,320 defendants executed since 1977 were innocent.”

The report notes another disturbing implication in capital murder cases:

“Fewer than half of all defendants who are convicted of capital murder are ever sentenced to death in the first place…Sentencing juries, like other participants in the process, worry about the execution of innocent defendants. Interviews with jurors who participated in capital sentencing proceedings indicate that lingering doubts about the defendant’s guilt is the strongest available predictor of a sentence of life imprisonment rather than death…It follows that the rate of innocence must be higher for convicted capital defendants who are not sentenced to death than for those who are. The net result is that the great majority of innocent defendants who are convicted of capital murder in the United States are neither executed nor exonerated. They are sentenced, or resentenced to prison for life, and then forgotten.”

In addition to Professor Gross, the authors of the study include Professor Barbara O’Brien, Ph.D, Michigan State University of Law; Chen Hu, Ph.D., Senior Biostatistician, American College of Radiology Clinical Research Center and Adjunct Scholar, Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine; and Edward H. Kennedy, Department of Biostatistics and Epidemiology, University of Pennsylvania School of Medicine.