The advent of computers and the internet—and the declining costs of using both—spurred law enforcement to construct massive databases of information about criminal offenders. Two prominent examples are DNA databases and sex offender registries. Both have been widely used since the 1990s, but they have different features that result in wildly different impacts on crime. DNA databases reduce recidivism and crime rates. Sex offender registries have no meaningful impact on either — and might even increase recidivism by registered offenders. Why the difference, and what does this tell us about how to use technology effectively to fight crime?
DNA databases aim to match known offenders with crime scene evidence. If a convicted offender or arrestee must provide a DNA sample — which depends on state law — authorities use a simple saliva swab to collect a sample, which is analyzed in a lab to generate a DNA profile (basically an identifying string of numbers akin to a Social Security number). Once uploaded to the database, an individual’s DNA profile is frequently compared with DNA profiles from crime scene evidence across the country, looking for matches that could point authorities to suspects in unsolved cases. As of February 2017, there were 12.8 million convicted offender profiles in CODIS — the national network of state DNA databases — and an additional 2.6 million profiles from arrestees.
Whether an individual is added to the database is known only to the individual and law enforcement; the information is not made public. But it appears that’s enough: Individuals know that once their profile is in the database, they’re much more likely to get caught if they commit another crime, and this knowledge deters them from reoffending.
In research published earlier this year, I compare individuals scheduled for release from prison just before and after a database expansion to test the effect of the DNA requirement on subsequent behavior. Those released before are the control group; those released after are the treatment group. These groups are nearly identical except that individuals in the treatment group are added to the DNA database and so are now more likely to be caught if they reoffend. I find that DNA profiling reduces recidivism by at least 17 percent for violent felony convicts and 6…