Newly published research examining 340 exonerations in the United States between 1989 and 2003 found that a significant number of those who were wrongly convicted had been sentenced to death. Researchers note that this finding appears to reflect two patterns: capital defendants are more likely to be convicted in error, and false convictions are more likely to be detected when defendants are on death row.
The paper, authored by Professor Samuel Gross of the University of Michigan Law School along with other assistants, reveals clear patterns associated with false convictions. The leading cause of wrongful convictions is perjury, including perjury by police officers, by jailhouse snitches, by the real killers, and by supposed participants and eyewitnesses to the crime who knew the innocent defedants in advance. The research revealed that false confessions, especially among vulnerable defendants such as juvenile offenders and those with mental retardation, also played a large role in murder convictions that led to exoneration. Almost all of the juvenile exonerees who falsely confessed were African American, and 90% of all exonerated juvenile defendants were African American or Hispanic.
Based on their review of these exonerations, the paper notes that "any plausible guess at the total number of miscarraiges of justice in American in the last fifteen years must run to the thousands, perhaps tens of thousands, in felony cases alone."