Asymmetrical Information - Megan McArdle

01.11.13

Can a DNA Database Prevent Crime?

Building better crime prevention, one data point at a time.

Everyone should read Kevin Drum's piece on the connection between environmental lead (mostly from gasoline) and the great decades-long crime wage of the postwar era.  (And Jim Manzi's appropriate cautions.)  I haven't blogged it mostly because I didn't have much to add: it seems persuasive.  Lead isn't good for you, and putting it into gasoline was a dangerous mistake.  

But I do have stuff to say about the US response.  The start of the crime wave coincided with extremely liberal policy towards crime, which attempted to address the root causes, and rehabilitate the criminal, rather than punish.  The fairly natural conclusion of the middle class in the 1970s and 1980s was that this was a bunch of bleeding-heart hooey, and that what we really needed was just to lock up a huge proportion of our population.  

To some extent this works just because people who are locked up can't commit any crimes.  And there's some deterrent effect, although not as much as you might think, because the chances of getting caught for any one crime are really pretty low. But of course, locking up the bottom income quintile would presumably reduce crime even more; doing so would round up most of the people who are committing crimes without getting caught.  

We don't do this because it would be insanely expensive, and also, insanely awful for all the people who hadn't done anything wrong.  But we should also remember that prison is insanely expensive and awful for people who have done something wrong.  Of course, for people reeling from massive public disorder and terrible violations, that was a feature, not a bug: making the punishment really awful both exacted retribution, and was supposed to deter crime.  

But there may be a better way to deter crime: instead of ratcheting up the punishment, you ratchet up the likelihood of getting caught.  This can decrease the expected value of crime, without wasting an entire human life behind bars.  

Luckily, we do seem to be getting a bit better at this.  The latest improvement, via Mark Kleiman, is DNA databases:    

Since 1988, every US state has established a database of criminal offenders’ DNA profiles. These databases have received widespread attention in the media and popular culture, but this paper provides the first rigorous analysis of their impact on crime. DNA databases are distinctive for two reasons: (1) They exhibit enormous returns to scale, and (2) they work mainly by increasing the probability that a criminal is punished rather than the severity of the punishment. I exploit the details and timing of state DNA database expansions in two ways, first to address the effects of DNA profiling on individuals’ subsequent criminal behavior and then to address the impacts on crime rates and arrest probabilities. I first show that profiled violent offenders are more likely to return to prison than similar, unprofiled offenders. This suggests that the higher probability of getting caught outweighs the deterrent effect of DNA profiling. I then show that larger DNA databases reduce crime rates, especially in categories where forensic evidence is likely to be collected at the scene—e.g., murder, rape, assault, and vehicle theft. The probability of arresting a suspect in new crimes falls as databases grow, likely due to selection effects. Back-of-the-envelope estimates of the marginal cost of preventing each crime suggest that DNA databases are much more cost-effective than other common law enforcement tools.  

Obviously, libertarians will worry that such databases invite abuse.  But so does 2.1 million people behind bars.