DNA databases will destroy the usefulness of DNA as a forensic tool

scienceI was listening to a very interesting KQED Forum program the other day on the new law in California that requires that anyone arrested has a DNA sample taken and stored in DNA database. As you would expect, the ACLU had quite a few issues with this new law and immediately filed suit to have it dismissed as unconstitutional. The ACLU lawyer made several points on how people can very easily be wrongly arrested and that there is no court involvement in deciding whether there is probable cause to take the sample. Also, they were quite concerned about the issue that once someone is in the database it’s incredibly difficult to get the sample expunged, even if the person is never charged with a crime.
As a counterpoint to the ACLU, Forum had a top California law enforcement official on to discuss how he felt about it as a policy. He took the position that innocent people had nothing to fear from being included in the database and that he would encourage his entire staff to submit samples. He also felt that the larger the database the better the chance the police had of being able to prosecute criminals in cold cases and who could possibly argue against catching criminals.
I believe that there is a major aspect that the guest representing law enforcement doesn’t see. In most cases currently, DNA is used to prove that a suspect was in a location.  Blood, semen, or hair samples are taken from a crime scene and then compared to a sample taken from the suspect. Scientists don’t actually look at the genome of the person, but compare a set of markers that if matched show a very high statistical likelihood that the samples came from the same person.  With the current setup there are a couple of issues that could arise, the scientists have to be very careful in following procedures so as to not contaminate the samples. Also with a batch of samples taken from a crime scene, samples that aren’t a match to suspects can be ignored as extraneous.
It’s with this last issue where having a massive database can cause havoc. Up until now, since extraneous samples were ignored there isn’t any incentive in planting false DNA. However, as a statistically relevant portion of the population enters the database this incentive changes. Now a criminal committing a murder can after wards pull out a DNA bomb and fire it at the crime scene. This could be a small device similar to a party popper, but instead of being loaded with confetti is loaded with random hair, blood, and semen samples. These could easily be obtained by offering criminals money to provide samples, since there’s little risk to them in this scenario. Now when the forensic scientists take samples at the crime scene and then run them through their PCR test equipment and compare them to the database, dozens if not hundreds of criminals will pop up as matches. The defense lawyers now have probable doubt, since how can law enforcement prove a negative in that non of these other criminals were the one to actually commit the crime.
DNA can work well and be trusted as a tool when the scope of the search is limited to the individual crime and when there aren’t large scale databases. If these databases are allowed to grow and more and more of the population is included we will start to see people being framed with DNA evidence. Unlike fingerprints, it’s much easier to plant DNA evidence and not that hard to obtain.