It has become an urban legend, at least among prosecutors, that jurors have been conditioned by the CSI franchise (CSI: Crime Scene Investigation, CSI: Miami, CSI: NY, and CSI: Cyber) to expect guilt to be proved beyond doubt by forensic evidence. Prosecutors fear that they will lose cases if they try to prove guilt the old-fashioned way: with confessions, eyewitness testimony, or inferences based on motive and opportunity.

It isn’t unreasonable for jurors to expect prosecutors to rely on expert witnesses when they can. After all, the Constitution requires guilt to be prove beyond a reasonable doubt. If fresh blood found at the scene of the crime isn’t analyzed for DNA, a jury might reasonably wonder why the government didn’t do everything in its power to determine whether the blood belonged to a criminal suspect.

Still, forensic evidence isn’t available in every case. Sometimes there’s no physical evidence to analyze. In other cases, budgets may be inadequate to fund an expert. When those circumstances occur, does a case become unwinnable?

Concerns About the CSI Effect

A state’s attorney in McLean County, Illinois recently warned readers of a Central Illinois newspaper that CSI is not realistic. When he questions potential jurors, he asks them whether they “expect to see satellite imagery and laser grid analysis.” His questions are intended to remind jurors that high tech evidence isn’t needed or available in every case.

Whether the CSI effect actually exists is open to debate. A 2006 survey asked participants whether they would expect to see various kinds of evidence in seven different cases ranging from murder to theft. Participants were asked, for example, whether they would expect to see “eyewitness evidence,” “some kind of scientific evidence,” “fingerprint evidence,” and “DNA evidence.”

The survey found that 46{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of participants expected to see some kind of scientific evidence in every criminal case, 32{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} expected to see ballistic evidence in every criminal case, and 22{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} expected to see DNA evidence in every criminal case. Since ballistic evidence pertains to firearms and since most crimes are committed without a firearm, the survey results might indicate that participants were not given sufficient information to make rational responses.

The survey also found that participants who watched a CSI show regularly were more likely to demand scientific evidence in every case than participants who rarely or never watched CSI. Those results were dutifully reported by the Central Illinois journalist who fretted about the impact of CSI on jurors.

The journalist neglected to report the survey’s central finding: Participants were not more likely to acquit a defendant simply because guilt was not supported by expert evidence. A recent article in the ABA Journal notes that the 2006 survey was followed by an urban survey in 2008-09 that found even less reason to believe that a CSI effect results in unwarranted acquittals. A judge who helped design the survey attributed worries about the CSI effect to “grumbling prosecutors.”

While survey participants said that the absence of scientific evidence would not make them more inclined to acquit, they were more likely to doubt guilt in the absence of eyewitness testimony. Ironically, eyewitness testimony is among the least reliable forms of evidence in a criminal prosecution. The defense lawyer in a case that turns on eyewitness identification should give automatic consideration to the need for an expert witness who can explain why identifications are so often mistaken.

Should Jurors Be Cautioned About the CSI Effect?

How participants respond to hypothetical questions on a survey is probably a poor measure of how they will judge the evidence in an actual trial. It makes little sense to think that jurors will worry about the absence of ballistic evidence in a case that does not involve a gun. No judge would allow a defense attorney to question the absence of evidence that isn’t relevant to the case.

The judge who was interviewed in the ABA Journal article argued that prosecutors might actually trigger a CSI effect by asking potential jurors whether they expect guilt to be proved by scientific evidence. If potential jurors aren’t thinking about scientific evidence before the prosecutor talked to them, they may wonder about its absence when the prosecutor calls attention to it.

In some cases, however, a defense lawyer’s comments about the prosecution’s failure to present forensic evidence is legitimate. The Journal article reports a Maryland case in which a criminal defense lawyer called the jury’s attention to the absence of scientific evidence to prove that the defendant had handled a screwdriver or tape found at the scene of the burglary.  No expert witness testified about fingerprints or DNA samples that one might expect to find on that evidence.

It is commonplace for lawyers to argue that a police investigation was incomplete and that the failure to make a diligent search for evidence creates reasonable doubt. The trial judge in the Maryland case nevertheless instructed the jury that ““there is no legal requirement that the state utilize any specific investigative technique or scientific test to prove its case.” That’s true, but there is a legal requirement that prosecutors prove guilt beyond a reasonable doubt.

An appellate court found that the instruction improperly undermined the defense theory that the absence of scientific evidence contributed to reasonable doubt. The defense lawyer did not argue that the prosecution had an obligation to present expert evidence. Rather, the lawyer argued that the prosecution had the ability to ask experts to analyze the evidence and that its failure to conduct a complete investigation of the evidence created a reasonable doubt about the defendant’s guilt. That argument did not warrant a jury instruction regarding the prosecution’s failure to produce scientific evidence.

Similar instructions have come to be known as “anti-CSI instructions.” Prosecutors argue that the instructions are needed to overcome the CSI effect. Since there is little evidence that the CSI effect actually exists, fair judges keep their thumb off the scale and avoid giving instructions that seem to excuse a prosecutor’s failure to introduce expert evidence when a case seems to call for it.