Elected officials often push for lengthening prison sentences for particular crimes in the hope of deterring people from committing them. But new research highlights a more effective and less costly approach: expanding databases that record the DNA of criminal offenders.

Law enforcement agencies have long maintained databases of convicted criminals’ fingerprints and mug shots. Such records help police to solve the many crimes that are committed by people with a criminal history. In some countries, such databases also sometimes include a DNA sample, typically obtained from saliva. When the Government of Denmark increased the proportion of arrestees from whom police gathered DNA samples from 4 per cent to 40 per cent in just five months, a research team took advantage of the natural experiment to evaluate whether DNA databases reduce crime.

In a study of more than 38,000 males who were arrested for crimes roughly equivalent in severity to felonies in the United States, the research team found that being added to the DNA database reduced reoffending by a stunning 43 per cent. The harsher criminal sentences that are favoured by self-styled “tough on crime” politicians have shown no evidence of producing a comparable level of crime reduction.

Because DNA evidence is found at many crime scenes — eg, most rapes and other violent assaults — DNA databases raise the odds that the offender will be identified and convicted. As has been shown in many other criminal justice programmes, raising the likelihood of being punished deters repeat offending far more than does increasing the severity of criminal penalties.

Study team member Jennifer Doleac, of the University of Virginia, said that even uneducated offenders who do not fully understand what DNA is nonetheless are deterred from recidivism after being added to the database because “television crime shows have a big impact on what people believe about the power of these tools”.

Denmark, of course, differs from the United States in many respects. But Doleac’s study of American DNA databases also found that they produce sharp drops in criminal reoffending. Her US work further notes that deterring a crime via DNA databases costs less than 10 per cent of the cost of producing the same deterrent effect by building more prisons. And because DNA-related technology is getting cheaper every year while prisons remain as costly as ever, that favourable cost-effectiveness ratio is only going to rise.

• Keith Humphreys is a Professor of Psychiatry at Stanford University and is an affiliated faculty member at Stanford Law School and the Stanford Neurosciences Institute