The Genetics of Innocence: Analysis of 194 U.S. DNA Exonerations

Abstract

This new analysis of 194 DNA exonerations, representing 171 criminal events, examines the types of evidence and DNA testing that have been used to free the victims of wrongful conviction. The types of DNA testing used to free the innocent parallels the growth of these techniques in forensic science. Short tandem repeat (STR) analysis now prevails (70%), though Y-STR analysis (16%) and mitochondrial testing (10%) are still used when STR analysis is not feasible, and the recently developed mini-STRs have been used for exonerations since 2008 (2.6%). The types of exculpatory evidence included intimate swabs (65%), clothing (53%), hair (13%), fingernail evidence (5%), cigarettes (3%), and other evidence. The most common factor associated with wrongful convictions was misidentification (75%), including misidentification by the victim (65%). False confessions (including admissions and pleas) were obtained in 30% of the cases, and informant testimony (including jailhouse and government informants) was used in 22% of the false convictions. Several types of invalid forensic science testimony were used to wrongfully convict in the 146 trials where transcripts or reliable forensic science data were available for analysis. Invalid testimony included serology (38%), hair comparison (22%), fingerprint comparison (2%), and bite mark comparison (3%). In 43% of the exonerations, the true perpetrator of the crime was identified through postconviction testing.