Biological Evidence in Adult and Adolescent Sexual Assault Cases: Timing and Relationship to Arrest

This study examined the timing of the crime laboratory report relative to arrests in sexual assault cases and explored the relationship between biological evidence and arrest in those cases in which the crime laboratory report came first and thus could have influenced the arrest decision. A random sample (N = 528) of cases that occurred between 2008 and 2010 and included a report to police was drawn from a Massachusetts statewide database of medical reports on sexual assault cases. Data from medical providers were merged with data abstracted from crime laboratory reports and with data requested from police departments. The vast majority (91.5%) of arrests took place before crime laboratory analysis could be conducted. The crime laboratory report was available before or near in time to the arrest in 11 cases. These cases were significantly more likely than other cases to have DNA profiles of the assailant, DNA matches to the suspect, and a match to another investigation in the FBI’s CODIS DNA database. Given that the probable cause needed to make an arrest in these cases was presumably established only after crime laboratory analysis was available, DNA may have helped lead to the arrest in these cases. However, these results should be interpreted very cautiously, because statistically significant results in early, small studies can have inflated effect sizes and often do not replicate in future studies. Because most arrests occur well before biological evidence is available, improvements in recovering biological evidence may have modest effects on arrest rates, though they may impact arrest rates by identifying more serial offenders. Future research on the relationship of biological evidence to arrest should use methods to increase sample size of relevant cases, such as oversampling cases with later arrests and using case control study designs. Future studies should also use case abstraction and interview methods to explore how police use biological evidence to make arrests.