By Bryan Porter

A prosecutor preparing a case has a very real conundrum to consider: How to deal with jurors who harbor unreasonable expectations about the government’s ability to present incriminating forensic evidence. In legal circles, this very real phenomenon is known as the CSI effect, given that television shows like “CSI:Miami” have been the source of so many misconceptions.

In the fantasy world of a television police procedural, a detective can run a DNA search on his handy portable tricorder and within seconds see a picture of the perpetrator underneath a flashing 24-point, bright red banner reading “MATCH.” Because a television investigation can only span about 44 minutes – a one-hour episode minus advertising – the detectives never conduct a fruitless crime scene search. Everything meshes together, and the investigation quickly produces an airtight case.

Not surprisingly, things are usually not that easy in the real world. Police departments and state-run forensic labs have limited budgets, and therefore have limited resources. Laboratory examination of potential evidence for the presence of DNA is both time-consuming and expensive. In serious cases such as murder, diligent efforts are made to conduct analyses likely to produce incriminating evidence. The fact remains, however, that it is impossible to exhaust all potential examinations. The defense can always argue that one more analysis should have been conducted.

More importantly, in the majority of investigations, no incriminating forensic evidence is located at all. On television, every examination attempted produces results. In reality, relevant DNA evidence is located in only about 10 percent of cases. The deposition of DNA is affected by a number of environmental factors, to include the temperature, level of humidity, length of time in situ, the surface on which the DNA was deposited and the relative oil content on the depositor’s skin. If the culprit was smart enough to wear gloves, it is almost impossible that his DNA was left at the scene.

Finally, it is axiomatic that in order to locate DNA evidence, the investigators must know where to look for it. Collectively, these factors make the identification of incriminating DNA from a crime scene the exception, not the rule.

Compounding this problem is that, on occasion, non-incriminating DNA evidence may be located. Imagine a scenario in which your home has unfortunately been burglarized, thankfully while you were away. The police determine that the burglar used the rear door to your home as his avenue of egress, breaking the lock in the process. Given that he likely touched the lock and door handle when entering your home, the police swab those areas for DNA, hoping to find the profile of the thief.

Whose DNA will be found on the door handle? Obviously, your profile could be there, as well as any other person living in the home. If you have children who play in the backyard, their DNA could be there. If your children had invited playdates over, their DNA – as well as that of their parents – could potentially be isolated. What if you had hosted a dinner party the week before? Any of the guests may have deposited their DNA on the back door. Since none of these people have felony criminal records, and therefore are not in the DNA database, if their DNA is recovered, the police will not be able to identify who deposited it.

Take the scenario one step further. The police later identify a suspect and arrest him. The suspect wore gloves during the crime and his DNA was, therefore, not discovered. However, an unidentified male DNA profile was isolated from the back door. In reality, that DNA was left by one of your dinner guests, but the police have no way of knowing that. Instead, the defense attorney is presented with an obvious argument for the jury: “Ladies and gentlemen, my client is innocent. His DNA was not located at the scene. Instead, some other man’s DNA was found on the back door – an unidentified profile that belongs to the real burglar.” A jury not weaned of the CSI effect may well agree with this line of reasoning.

For that reason, prudent prosecutors must spend quite a bit of time discussing the limitations of real-world crime scene investigations to try to inoculate their cases from unreasonable juror expectations.

The writer is the Commonwealth’s Attorney for Alexandria.