Sexual Assault and Murder: When DNA Does Not Help Even Though It Is Present

Abstract

Two cases demonstrate that full reliance on biological stains (“CSI effect”) may, in complicated or unlikely case scenarios, lead to either wrongful convictions or no prosecution at all. Case 1 was a homicide in a small town in which the short tandem repeat (STR) profile of the neighbor of the deceased was found on two fingers of the dead person. No motif for the crime was established. Transfer of DNA through newspapers was excluded even though the neighbors did share newspapers on a daily basis. We performed experiments and found an unexpected solution for the DNA findings. In Case 2, a woman reported that she was anally raped for about 1 hour in her bed while her parents and grandfather were in the same house. The police and DA’s office of the larger city dismissed the case as improbable even though hematoma and anal fissures were documented in the hospital and the woman audio-recorded the event on her cell phone. On the oral, vaginal, and anal swabs, no or hardly any autosomal STRs of the man were found which led to a non-prosecution. On inspection of her pantyhose, we found clear signals from Y-STRs, though. We discuss structural problems that may occur even in highly standardized laboratories if wrong assumptions about – unlikely yet true – events enter the case files.