Connecting the Dots: Identifying Suspected Serial Sexual Offenders Through Forensic DNA Evidence

Introduction: Most sexual assaults that are reported to the criminal justice system will not be prosecuted. Researchers and policymakers have expressed concern that this long-standing practice allows offenders to commit additional sexual assaults. Determining whether reported sex offenders commit other sexual assaults requires establishing reliable linkages between two (or more) cases. Typically, criminal history records are used to identify repeat sexual offenders, but biological evidence in sexual assault kits (SAKs; also termed rape kits) provides an another way to study how often reported sexual offenders commit additional sexual assaults by linking DNA across multiple cases to the same perpetrator. Method: This study examined the forensic DNA testing results from a large sample of SAKs from Detroit, Michigan (N = 7,287). We assessed how many SAKs yielded a DNA match to a reference sample in the federal criminal database CODIS (Combined DNA Index System). We then ascertained whether the matching case was related to another sexual assault incident documented by state criminal history records. Results: Approximately one third (35.7%) of the unique perpetrators in this sample had two or more sexual assaults linked via DNA, which is higher than what is typically documented in recidivism studies using court records (8-15%). Three case studies are presented that highlight how forensic DNA evidence can link multiple sexual assaults to the same perpetrator. Conclusion: Forensic DNA testing of SAKs reveals a more complete picture of the scope of offenders' sexual perpetration behaviors than what is documented in criminal history records alone.