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Does DNA evidence actually increase the odds of conviction?

As more criminal justice systems increase reliance on DNA databases as crime solving tools, researchers are regularly evaluating database efficacy. A study published on February 10, 2022, “The Relationship of DNA Evidence to Prosecution Outcomes in Sexual Assault Cases”, found that when DNA evidence is available in a sexual assault case, prosecutors are more likely to pursue criminal charges, and the odds of conviction are nine times greater than in similar cases without DNA evidence (Cross 2022).

Published earlier this year, the study analyzed a sample of 257 sexual assault cases that were referred to a metropolitan prosecutor’s office in Massachusetts by law enforcement. The study controlled for other variables that could potentially impact the rates of prosecution, such as presence of additional evidence. The basis of the study intended to capture the influence of DNA evidence on the outcome of prosecuting sexual assault cases. Due to the limitations of the study as conducted, it is unlikely that these findings can be attributed to other variables.

Convictions in cases of sexual assault can be onerous due to difficulties in the process of identifying the assailant. Previous studies have found that a DNA match may be the only effective method to clearly establish when a sexual assault is committed by a stranger to the victim. It is therefore crucial for DNA samples to be collected to help increase chances that the assailant can be identified.

The study results are framed to incentivize prosecutors to actively seek out DNA evidence prior to prosecuting a sexual assault case. Occasionally, a DNA profile will be available if the suspect is a repeat offender, but DNA evidence is not present in all sexual assault cases. Prosecutors are often able to obtain a biological sample from a suspect if authorized through a court order, but this extra step is often at the discretion of the prosecutor and is not automatic in each case. Given that less than one-tenth of cases where analysis at a crime laboratory resulted in no DNA match and less than one-fifth of cases not analyzed at a crime laboratory at all resulted in a conviction, it is clear that DNA evidence plays a significant role in a trial. Over half of the cases with a DNA match to suspect ended in conviction, demonstrating to prosecutors that securing DNA evidence is crucial in creating a case.

Study: Cross TP, Siller L, Vlajnic M, Alderden M. The Relationship of DNA Evidence to Prosecution Outcomes in Sexual Assault Cases. Violence Against Women. February 2022. doi:10.1177/10778012221077124


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