Suspect identification using DNA evidence has become a common tool in modern criminal investigations, enabling law enforcement to identify offenders, connect cases, and strengthen evidence used in prosecution, particularly for serious violent crimes including sexual assault and homicide. Over the past two decades, the Combined DNA Index System has grown into a national infrastructure that allows jurisdictions to compare DNA profiles and generate investigative leads across cases.

As the use of DNA spreads, researchers have sought to answer a core question: how valuable is this evidence in criminal investigations? The answer is straightforward. As DNA databases grow, they become more effective in two ways:

1. Research shows that the same forensic evidence produces more investigative leads over time simply because there are more profiles to match against. In practical terms, expanding eligibility for DNA collection, whether at arrest or conviction, can increase the likelihood that crime-scene evidence leads to a suspect.
2. As more suspects are identified, the perceived likelihood of detection increases, deterring crimes.

At the individual level, inclusion in a DNA database is also associated with lower reoffending rates. Studies in both the United States and Denmark find that individuals subject to DNA collection are less likely to be convicted of new crimes, particularly in the period immediately following inclusion. The effects appear strongest among younger people and those earlier in their criminal histories, suggesting that the timing of database inclusion may matter.

The impact of DNA use in criminal investigations extends beyond individual cases. Research suggests that larger DNA databases are associated with reductions in both violent and property crime. These findings are consistent with a deterrence effect: when individuals believe they are more likely to be identified through DNA, they may be less likely to offend. 

As DNA databases expand, they increase both investigative capacity and the perceived likelihood of detection, two factors that can contribute to improved public safety.

Beyond its deterrent effects, DNA evidence can directly improve case outcomes, particularly in high-volume crimes such as burglary. A randomized controlled trial I conducted with colleagues showed that integrating DNA testing into property crime investigations significantly increased rates of suspect identification, arrest, and prosecution compared to traditional criminal investigations. Importantly, DNA-aided investigations, on average, identify individuals with more extensive criminal histories than conventional investigations, suggesting that DNA is effective in linking crime scene evidence to serious, chronic offenders.

As evidence for the effectiveness of DNA-aided investigations grows, many states are considering legislation to collect DNA samples earlier in the criminal justice process. These policy decisions about DNA expansion require careful attention to fairness and civil liberties. Differences in arrest, conviction, and incarceration patterns mean that eligibility rules for DNA collection can affect which populations are represented in DNA databases. Expanding who is lawfully required to provide DNA may improve effectiveness, but it also raises important questions about privacy, consent, and fairness.

Research suggests that policymakers need to take a balanced, evidence-based approach to expanding DNA databases:

• Carefully considering how eligibility rules affect effectiveness and representation across populations
• Considering when to invest in DNA collection and processing to strengthen investigative outcomes, especially in high-volume cases
• Exploring strategies that increase awareness of database inclusion to enhance deterrence

Maintaining clear safeguards is essential to protect civil liberties and sustain public trust, and continued research is needed to assess the cost-effectiveness of current and emerging technologies and practices. Moving forward, policymakers, practitioners, and researchers need to work together to implement and evaluate these approaches to ensure DNA is used as effectively and responsibly as possible to improve public safety.