Factors associated with trace evidence analyses and DNA findings among police reported cases of rape

The aim of this study was to examine the association between victim, suspect and assault characteristics and (1) forensic analysis of trace evidence, (2) detection of spermatozoa and (3) DNA match in police-reported cases of rape/attempted rape. In addition, we explored whether DNA findings were associated with legal outcome.

We conducted a retrospective, descriptive study based on police-reported rapes and attempted rapes of women  ≥16 years of age in Sør-Trøndelag Police District throughout 1997-2010. Police data were merged with information from the Sexual Assault Centre (SAC) at St. Olavs University Hospital, Trondheim, Norway. We used binary and multivariable logistic regression for the comparisons.

We identified 324 victims (mean age 24 years). The police requested analysis in 135 (45%) of the 299 collected victim samples. The police decision to analyze was after adjustment associated with the victim being employed or under education, and a public venue, but not with interval from assault to sampling. Spermatozoa were detected in 79 (61%) of the analyzed cases, of which 71 were collected from victims within 24h. Interval from assault being <24h and reporting a penetrative assault remained associated with the findings of spermatozoa after adjustments. Forensic analyses of trace evidence collected from victim, suspect and/or venue disclosed matching DNA profiles in 57 (40%) of a total of 143 analyzed cases. Matching DNA profiles were associated with suspect being known to the victim and with the venue being private. A higher proportion of cases with a DNA match were prosecuted in court: 20 of the 29 cases prosecuted. However, despite a DNA match 35 cases were anyway dismissed because of insufficient evidence.

Although many of the associations in our study were expected, it is still important to report the actual numbers to gain insight into the importance of a DNA match in legal proceedings. A substantial proportion of cases with DNA match was dismissed because of insufficient evidence. To strengthen the justice response to sexual assault, it is essential to generate knowledge about the role of medico-legal evidence in such cases, and there are obviously other non-medical factors influencing the legal decisions.