On our blog last week, we began exploring the notion that some of the "evidence" in criminal cases is not quite as "scientific" as it seems. Often, juries see certain types of evidence--bite marks, blood spatter, hair analysis, for example--as irrefutable scientific proof that a defendant committed a crime. Instead, we see that this evidence is often based in unproven scientific theory rather than fact. What seems like proof to a jury may just be a lot of filled in blanks and assumptions presented by a self-described forensic expert.

One type of evidence that is often used to help secure a criminal conviction is fingerprint analysis. It would seem, on the surface, as if fingerprint analysis would be a sure-fire method of identification. After all, no two fingerprints are alike, right? Although there are some problems with making that assumption, the greater issue lies in the fact that detectives rarely find a full, intact print at the scene of a crime. Instead, they must rely on partial prints and "expert" analysis.

In analyzing fingerprints, forensic experts examine a partial print to determine how many points, defined by ridges and whorls of the fingerprint, match with the fingerprint of a suspect. But even among experts, there is disagreement about how many points must correlate between the two in order to determine a "match." Some experts say a match can be determined by matching as few as 12 points; others say 20 points is necessary to determine a match. If experts can't agree on what makes a definitive match, how can juries be expected to determine the answer?

Furthermore, even if full, complete fingerprint was lifted from a crime scene, even if the print could be proven beyond all doubt to belong to a suspect, the print itself does not mean the suspect had anything to do with the crime. It only proves that he or she was in that location at some point in time, possibly days or even weeks before the crime occurred.

Certainly, fingerprint analysis can be useful in helping law enforcement to identify a suspect, but it is hardly the irrefutable scientific evidence that proves a defendant committed a crime. In fact, some federal courts have ruled that fingerprint evidence fails to meet the United States Supreme Court's standards of scientific evidence, known as the Daubert trilogy:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

Most courts, however, allow expert witnesses to testify whether or not a fingerprint discovered as evidence is considered to be a match. It is up to the jurors to determine how reliable they believe that testimony and evidence to be.

As ingrained as fingerprint evidence is as a part of criminal investigation and prosecution, it seems unlikely that courts will completely exclude it as admissible evidence any time soon. In order to change public perception of fingerprint evidence as undeniable proof, it will likely take a glaring and obvious destruction of the theories of fingerprint analysis to change things.

Image Credit: Chad Miller