Television shows and movies about CSI often seem to present a science fiction version of investigation that involves amazing technology and incredible inferences. While people do get that the almost magical solving of crimes is fiction, there is still considerable overconfidence in many methods used in real investigations. This overconfidence plays a significant role in some of the problems infecting the criminal justice system.
The history of criminal investigation is replete with debunked methods, such as the use of phrenology to diagnose criminal tendencies. There are also technologies that have little or no validity and are not admitted in court, yet enjoy some public confidence (such as lie detectors). There are also methods that might have some value in investigations, yet are the subject of unwarranted overconfidence in their efficacy. These include such things as bite mark analysis and fiber analysis. Other methods are reasonable useful, such as fingerprints, yet are still often accepted with an unwarranted level of confidence—especially in situations where the defendant has an ill-prepared and overworked public defender. Defendants of means or fame can, of course, purchase a better sort of justice.
In contrast with the above methods, DNA identification strikes many as a silver bullet. After all, aside from identical twins (or clones), no two people have the same DNA. This would seem to make the presence of a person’s DNA at a crime scene extremely good evidence for their involvement.
While such evidence is valuable, it is rather important to consider the limitations of and problems with this method. Contamination and transference should always be given due consideration because DNA can travel quite far. This can be illustrated with an example involving my husky.
Like all huskies, my husky generates an incredible amount of fur and this fur gets onto everything and everyone. The fur is thus transported from my house to various points around the world—I know for a fact that her fur is now in at least five states—although she has not left Florida. As such, if fur sampling was used to determine what dogs had been present, she could show up as being present in many, many locations that she did not visit. The same also holds true for humans. While humans do not shed like huskies, humans do shed hair and this can get onto people and objects that could end up in crime scenes. For example, if Sally wears a hat and it ends up in someone else’s possession, that hat will almost certainly still have Sally’s DNA on it. So, if the hat is found at a crime scene, Sally’s DNA will be found there as well, which could be trouble for Sally.
These concerns do not show that DNA testing should not be used; rather they show that is wise to maintain a degree of healthy skepticism in the face of such evidence. It also shows the importance of informing law enforcement, judges, juries and lawyers about the limitations of methods This assumes, of course, that those involved (have the time to) care about justice—which is not always the case in the criminal justice system. There is also the concern, as noted above, that the quality of a person’s defense is a function of their available resources (be they money or fame). These are, of course, concerns that go far beyond worries about particular methods.
It can be objected that educating people about the limits of such methods could create a skepticism that might undermine convictions. For example, that the possibility of “wandering DNA” could be used to create unwarranted doubt, thus allowing the guilty to go free. A skilled and well paid lawyer could exploit such doubts quite effectively and allow a lawbreaker to go free—thus preventing justice from being done.
This concern is reasonable; while overconfidence is problematic, so is under-confidence. However, the United States’ criminal justice system is supposed to operate on a presumption of innocence: it is better to err on allowing the guilty to go free than to err towards punishing the innocent. As such, the greater mistake would be overconfidence in a method. However, there is the concern that these doubts would be exploited by those who have the resources to purchase an effective defense, while the less fortunate would not benefit from them. But, as has been noted, this is a general problem with America’s pay-to-play legal system.