DNA may have only modest impact on sexual assault arrests

Most arrests in sexual assault cases occur before crime laboratory results are available, a new study found, suggesting that DNA testing may influence arrests in just a small number of cases.

The paper is among the first studies to examine the impact of DNA on sexual assault arrest rates, although the researchers said the findings should be interpreted cautiously because of the small sample size of cases used in key analyses.

In the study, researchers examined biological evidence in 528 sexual assault cases that occurred in Massachusetts between 2008 and 2010. Drawn from a statewide database, the sample included incidents involving a victim age 12 or older on which a medical examination was performed and a crime laboratory report was provided to police.

The vast majority of arrests - more than 91 percent - occurred before biological evidence was processed by crime laboratories and reports were available to police investigators, according to lead author Theodore P. Cross from the University of Illinois.

"When arrests are made, they're generally made fairly quickly, most within a day or two based on the victim report and the police investigation," Cross said. "There was only a small sample of arrests - slightly more than 2 percent - that occurred after the crime lab was able to analyze the biological sample. This illustrates some of the limitations of biological evidence because most arrests really do occur well before the crime lab analyzes the data."

Most of the evidence kits reached the crime laboratory about eight days after victims' examinations. The lab's median turnaround time from the kits' arrival until the results were reported to police was 43 days, the researchers found.

The findings suggest that "improvements in recovering biological evidence may have only modest effects on arrest rates, except in a small number of cases in which DNA evidence may become the determining factor because assailants cannot be identified without it," Cross said.

Biological evidence is rarely used alone to substantiate a case but rather as one component in an array of evidence, Cross said. "We need to do more of a process analysis on those cases in which biological evidence really does have an impact to understand better how police and prosecutors really use it."

If advances in recovering and extracting biological samples enable the identification of more serial perpetrators, biological evidence could begin to have a more significant impact on arrest rates in the future, Cross said.

"I think our findings help justify the investment that's been made in forensic technology, but they also make it clear the biological evidence is not going to be a panacea, and you need to make investments in other areas that could improve the criminal justice response."

The study is published online by the Journal of Interpersonal Violence.